Categorical max_row_index, max_n_row_index and min equivalents

[ianthomas23]

This is part two of issue #1210. It adds support for reductions like these:

• by("cat", _max_row_index())
• by("cat", _min_row_index())
• by("cat", _max_n_row_index(n=3))
• by("cat", _min_n_row_index(n=3))

on CPU and GPU, with and without dask.

The remainder of #1210 will be a single larger PR to provide support for by-where reductions such as

• by("cat", where(max("value"), "other"))

[codecov[bot]]

Codecov Report

Merging #1233 (4cf7770) into main (f917cd9) will decrease coverage by 0.03%. The diff coverage is 64.55%.

@@            Coverage Diff             @@
##             main    #1233      +/-   ##
==========================================
- Coverage   83.59%   83.56%   -0.03%     
==========================================
  Files          35       35              
  Lines        8751     8773      +22     
==========================================
+ Hits         7315     7331      +16     
- Misses       1436     1442       +6     

Impacted Files	Coverage Δ
datashader/transfer_functions/_cuda_utils.py	20.63% <0.00%> (ø)
datashader/reductions.py	78.86% <57.14%> (-0.17%)	:arrow_down:
datashader/compiler.py	88.54% <100.00%> (+0.12%)	:arrow_up:
datashader/utils.py	81.72% <100.00%> (+0.09%)	:arrow_up:

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