enh: add "multipart DWI" acquisitions and refactor DWI specifications

Addresses the use case of DWI sequencies that need to be split in several
runs because, e.g., the scanner would exceed temperature specifications
due to fast gradient-switching.

This PR also refactors the text of DWIs to better accomodate the addition
and make DWIs more consistent with the rest of the file.

The approach to solve this is similar to that of bids-standard#622 for fieldmaps.

References: nipreps/dmriprep#43.

src/04-modality-specific-files/01-magnetic-resonance-imaging-data.md

@@ -373,6 +373,13 @@ participant, task and run takes precedence.
 
 ## Diffusion imaging data
 
+Currently supported image modalities include:
+
+| **Name**              | `modality_label` | **Description**                                                                                                                                     |
+|---------------------- | ---------------- | --------------------------------------------------------------------------------------------------------------------------------------------------- |
+| DWI                   | dwi              | Diffusion-weighted imaging contrast (specialized T2\* weighting)                                                                                     |
+| Single-Band Reference | sbref            | Single-band reference for one or more multi-band `dwi` images, only when no gradient informations needs to be stored (i.e., all volumes are *b = 0*) |
+
 Template:
 
 ```Text
@@ -386,25 +393,33 @@ sub-<label>/[ses-<label>/]
        sub-<label>[_ses-<label>][_acq-<label>][_dir-<label>][_run-<index>]_sbref.json
 ```
 
-Diffusion-weighted imaging data acquired for that participant. The OPTIONAL
-[`acq-<label>`](../99-appendices/09-entities.md#acq)
+Diffusion-weighted imaging data acquired for that participant.
+
+The OPTIONAL [`acq-<label>`](../99-appendices/09-entities.md#acq)
 key/value pair corresponds to a custom label the user may use to
-distinguish different set of parameters. For example this should be used when a
-study includes two diffusion images - one single band and one multiband. In such
-case two files could have the following names:
+distinguish different sets of parameters.
+For example, this should be used when a study includes two diffusion
+images -- one single band and one multiband.
+In such case two files could have the following names:
 `sub-01_acq-singleband_dwi.nii.gz` and `sub-01_acq-multiband_dwi.nii.gz`,
 however the user is free to choose any other label than `singleband` and
-`multiband` as long as they are consistent across subjects and sessions. For
-multiband acquisitions, one can also save the single-band reference image as
-type `sbref` (e.g. `dwi/sub-control01_sbref.nii[.gz]`) The bvec and bval files
-are in the [FSL format](https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FDT/UserGuide#DTIFIT):
+`multiband` as long as they are consistent across subjects and sessions.
+
+For multiband acquisitions, one can also save the single-band reference image as
+type `sbref` (e.g. `dwi/sub-control01_sbref.nii[.gz]`.)
+
+
+### REQUIRED gradient orientation information
+
+The bvec and bval files are in the
+[FSL format](https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/FDT/UserGuide#DTIFIT):
 The bvec files contain 3 rows with n space-delimited floating-point numbers
-(corresponding to the n volumes in the relevant NIfTI file). The first row
-contains the x elements, the second row contains the y elements and third row
-contains the z elements of a unit vector in the direction of the applied
-diffusion gradient, where the i-th elements in each row correspond together to
-the i-th volume with `[0,0,0]` for non-diffusion-weighted volumes. Inherent to
-the FSL format for bvec specification is the fact that the coordinate system of
+(corresponding to the n volumes in the relevant NIfTI file).
+The first row contains the *x* elements, the second row contains the *y* elements and
+the third row contains the *z* elements of a unit vector in the direction of the applied
+diffusion gradient, where the *i*-th elements in each row correspond together to
+the *i*-th volume, with `[0,0,0]` for non-diffusion-weighted volumes.
+Inherent to the FSL format for bvec specification is the fact that the coordinate system of
 the bvecs is with respect to the participant (i.e., defined by the axes of the
 corresponding dwi.nii file) and not the magnet's coordinate system, which means
 that any rotations applied to dwi.nii also need to be applied to the
@@ -432,6 +447,53 @@ bval example:
 and thus define those values for all sessions and/or subjects in one place (see
 Inheritance principle).
 
+## Multipart (split) DWI schemes
+
+Some MR schemes cannot be acquired directly by some scanner devices,
+requiring to generate several DWI runs that were originally meant to belong
+in a single one.
+For instance, some GE scanners cannot collect more than &asymp;160 volumes
+in a single run under fast-changing gradients, so acquiring *HCP-style*
+diffusion images will require splitting the DWI scheme in several runs.
+
+| **Key name**    | **Requirement level** | **Data type** | **Description**                                                                                                                                                                               |
+| --------------- | --------------------- | ------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| MultipartID     | REQUIRED              | [string][]    | Unique identifier to prescribe DWI segments belonging in a single multipart run.                                                                                                              |
+
+JSON example:
+
+```JSON
+{
+  "PhaseEncodingDirection": "j-",
+  "TotalReadoutTime": 0.095,
+  "MultipartID": "dwi_protocol1"
+}
+```
+
+For instance, the above JSON sidecar defined for a set of files via the
+`sub-<label>_dwi.json` file would indicate that all the runs 1 through 4
+belong in the same unique image:
+
+```Text
+sub-<label>/[ses-<label>/]
+    dwi/
+       sub-<label>_run-1_dwi.nii.gz
+       sub-<label>_run-1_dwi.bval
+       sub-<label>_run-1_dwi.bvec
+       sub-<label>_run-2_dwi.nii.gz
+       sub-<label>_run-2_dwi.bval
+       sub-<label>_run-2_dwi.bvec
+       sub-<label>_run-3_dwi.nii.gz
+       sub-<label>_run-3_dwi.bval
+       sub-<label>_run-3_dwi.bvec
+       sub-<label>_run-4_dwi.nii.gz
+       sub-<label>_run-4_dwi.bval
+       sub-<label>_run-4_dwi.bvec
+       sub-<label>_dwi.json
+```
+
+## Other RECOMMENDED metadata
+
 See [Common metadata fields](#common-metadata-fields) for a list of
 additional terms that can be included in the corresponding JSON file.