feat: add geneset initialization

[atarashansky]

Genesets are stored as a JSON blob in the obs tiledb metadata under key "genesets". The new endpoint added to the server simply forwards this blob to the client.

The client added back geneset initialization from CELLxGENE Annotate.

Each cell type present in a CellGuide CXG has its own marker gene set from CellGuide.

Also added some QOL improvements to the categorical data:

• Ordering of labels in decreasing order by mean expression when coloring by continuous metadata
• Removal of cells with 0 labels when subsetting/unsubsetting
• Unsubset and subset hotkeys

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 77.47%. Comparing base (bd4f07a) to head (0943d1b).

❗ Current head 0943d1b differs from pull request most recent head bf18b89. Consider uploading reports for the commit bf18b89 to get more accurate results

Files	Patch %	Lines
server/dataset/cxg_dataset.py	27.27%	8 Missing ⚠️
server/common/rest.py	28.57%	5 Missing ⚠️
server/app/api/v3.py	83.33%	1 Missing ⚠️
server/dataset/dataset.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #803      +/-   ##
==========================================
- Coverage   77.74%   77.47%   -0.27%     
==========================================
  Files          88       88              
  Lines        6802     6856      +54     
==========================================
+ Hits         5288     5312      +24     
- Misses       1514     1544      +30     

Flag	Coverage Δ
frontend	77.47% <44.44%> (-0.27%)	⬇️
javascript	77.47% <44.44%> (-0.27%)	⬇️
smokeTest	∅ <ø> (∅)
unitTest	77.47% <44.44%> (-0.27%)	⬇️

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[seve]

Just a couple items for discussion

[tihuan]

PR looks great! Thanks for all the code comments to provide the context, Alec 🥇 ! Just some nits otherwise LGTM!