Add 'spatial' extra install

[ivirshup]

Make it possible to do pip install "cellxgene_census[spatial]"

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.42%. Comparing base (50fea93) to head (98b68aa).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1346   +/-   ##
=======================================
  Coverage   91.42%   91.42%           
=======================================
  Files          82       82           
  Lines        6487     6487           
=======================================
  Hits         5931     5931           
  Misses        556      556           

Flag	Coverage Δ
unittests	91.42% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.