Again more docstrings

[marcpaterno]

This updates more docstrings (and a few type annotations).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.4%. Comparing base (bd70476) to head (e9c8ed6).
Report is 16 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #447   +/-   ##
======================================
  Coverage    98.4%   98.4%           
======================================
  Files          48      48           
  Lines        3666    3666           
======================================
  Hits         3611    3611           
  Misses         55      55           

Files with missing lines	Coverage Δ
firecrown/likelihood/number_counts.py	91.1% <ø> (ø)
firecrown/likelihood/source.py	100.0% <100.0%> (ø)
firecrown/likelihood/statistic.py	91.1% <100.0%> (ø)
firecrown/likelihood/student_t.py	100.0% <100.0%> (ø)
firecrown/likelihood/supernova.py	100.0% <ø> (ø)

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