Separate where combine_cpu functions by ndim

[ianthomas23]

This separates out the where reduction combine_cpu functions by number of array dimensions, which may be 2, 3 or 4 depending on whether they contain categorical and/or a *_n selector reductions. This is the same as already done for the combine_cuda functions.

[codecov]

Codecov Report

Merging #1265 (3edd83a) into main (7e3b8c3) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1265      +/-   ##
==========================================
+ Coverage   83.44%   83.47%   +0.03%     
==========================================
  Files          35       35              
  Lines        9071     9089      +18     
==========================================
+ Hits         7569     7587      +18     
  Misses       1502     1502              

Files Changed	Coverage Δ
datashader/reductions.py	80.34% <100.00%> (+0.24%)	⬆️

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