Adds EPA NSIM stats and precision functions

[dhhagan]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.50746% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 73.39%. Comparing base (450aed8) to head (c4fbac4).
Report is 6 commits behind head on main.

Files	Patch %	Lines
atmospy/stats.py	97.61%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   69.01%   73.39%   +4.38%     
==========================================
  Files          11       13       +2     
  Lines         384      451      +67     
==========================================
+ Hits          265      331      +66     
- Misses        119      120       +1     

Flag	Coverage Δ
unittests	73.39% <98.50%> (+4.38%)	⬆️

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