Use a naive sparse histogram.

[delucchi-cmu]

We really just need the non-zero indexes and non-zero values, and the complexity of the general scipy sparse array is adding a lot of compute overhead, primarily in the catalog import "Binning" stage.

astronomy-commons/hats#446

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.57%. Comparing base (1aa15a8) to head (c559473).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #463      +/-   ##
==========================================
- Coverage   94.59%   94.57%   -0.02%     
==========================================
  Files          28       28              
  Lines        1627     1622       -5     
==========================================
- Hits         1539     1534       -5     
  Misses         88       88              

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