Drop z dimension from ImageStack DataArray before shading

[philippjfr]

When performing categorical rasterization with only one category the XArrayInterface would convert the 3D DataArray into a Dataset. When this 3D Dataset was then shaded we would convert the singular DataArray variable into a z-dimension, which meant we'd have two categorical variables (with the same value) and datashader would error out because it expects a 2D or 3D array.

Here we do a number of things:

• Allow the XArrayInterface to hold a DataArray with a singular value dimension
• Ensure that shade handles non-xarray datasets correctly (by promoting them to xarray)
• Ensure that it does not needlessly add a z-dimension if we have a singular value dimension
• Even if we have a 3D array and a multiple z-values we squeeze the DataArray to drop superfluous constant dimensions.
• Support dask array backed xarrays in shade (by materializing them)

[codecov]

Codecov Report

Attention: Patch coverage is 82.75862% with 5 lines in your changes missing coverage. Please review.

Project coverage is 88.49%. Comparing base (88996ed) to head (f108020).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/operation/datashader.py	61.53%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6378      +/-   ##
==========================================
- Coverage   88.50%   88.49%   -0.01%     
==========================================
  Files         323      323              
  Lines       68281    68304      +23     
==========================================
+ Hits        60429    60448      +19     
- Misses       7852     7856       +4     

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[hoxbro]

If you haven't already maybe also run the hvplot test suite (locally) to verify that this change does not break anything because of the changes in XarrayInterface.

[philippjfr]

If you haven't already maybe also run the hvplot test suite (locally) to verify that this change does not break anything because of the changes in XarrayInterface.

Done, it's passing.