Support summary containing by reduction with other reductions

[ianthomas23]

Fixes #1256.

Adds support for the following combinations of summary and by reductions:

1. summary(by("cat1"), other_reduction) and with the order swapped.
2. summary(by("cat1"), by("cat1"))
3. summary(by("cat1"), by("cat2"))

The biggest change here is in the handling of categorical columns in compiler.py make_append(). Previously the categorical flag was constant for all reductions within a summary, now it is checked for each constituent reduction. Where categorical columns are reused, they are only extracted once.

Also, the addition of kwargs to xr.DataArray in by.finalize has been modified to follow the same approach as in where reductions, which is to add the new categorical coords/dims to a copy of the kwargs, not modify them in-place. Hence each constituent reduction of a summary can have different categorical coordinates from others.

Tested on CPU and GPU, with and without Dask.

[codecov]

Codecov Report

Merging #1257 (e45fee4) into main (4a1c3fc) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1257      +/-   ##
==========================================
- Coverage   83.44%   83.41%   -0.04%     
==========================================
  Files          35       35              
  Lines        8882     8886       +4     
==========================================
  Hits         7412     7412              
- Misses       1470     1474       +4     

Impacted Files	Coverage Δ
datashader/compiler.py	88.55% <100.00%> (+0.17%)	⬆️
datashader/reductions.py	77.80% <100.00%> (-0.27%)	⬇️

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