Verify MRtrix bvecs/bvals import

[Lestropie]

In the first committed version of code, the scope of testing may be limited to the conversion from DICOM to some other format, and checking of phase encoding direction / slice encoding direction / gradient table.

The scope of these tests will need to be expanded to cover:

• MRtrix3 import of bvecs / bvals, particularly dependent on RealignTransform (mrconvert: fslgrad and RealignTransform: 0 MRtrix3/mrtrix3#2477)
• MRtrix3 export of bvecs / bvals when alternative strides are specified