Enable categorical subsets

[jfoster17]

Pull Request Template

Description

This is a work-in-progress towards providing the ability to automatically create subsets for categorical attributes (as suggested in #1151 and #2004). Ultimately, I envision this might expand to allow subsetting on up to two categoricals at once, and for this reason I have added it as a new dialog option instead of modifying the existing facet_subset. Some items that remain:

• Alert the user if they try to make subsets from a categorical attribute with too many categories
• Add additional qualitative colormaps appropriate for categorical subsets. Currently I just added a single qualitative colormap to the ColormapRegistry, but this is probably not the best solution.
• Tests
• (possibly) Expand to allow creating all combinations of up to two categoricals (e.g. for examining combinations experimental factors)

[codecov]

Codecov Report

Merging #2288 (8b0eeef) into main (e8f03f1) will decrease coverage by 0.15%.
The diff coverage is 36.61%.

@@            Coverage Diff             @@
##             main    #2288      +/-   ##
==========================================
- Coverage   88.13%   87.97%   -0.16%     
==========================================
  Files         247      247              
  Lines       23292    23360      +68     
==========================================
+ Hits        20528    20551      +23     
- Misses       2764     2809      +45     

Impacted Files	Coverage Δ
glue/core/util.py	71.98% <4.54%> (-5.71%)	⬇️
glue/dialogs/subset_facet/qt/subset_facet.py	71.59% <47.05%> (-21.27%)	⬇️
glue/app/qt/layer_tree_widget.py	79.82% <57.14%> (-0.78%)	⬇️
glue/config.py	85.11% <100.00%> (+0.03%)	⬆️
glue/conftest.py	67.50% <0.00%> (+3.75%)	⬆️

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[astrofrog]

@jfoster17 - are you still interested in pushing this forward? If so, this will need a rebase - thanks!