Add sum to stats module

[psavery]

Overview

This generalizes the logic for average and sum to be in an _accumulate function which also takes the output type as a parameter.

This allows average() output to remain as an np.float32, and the new sum() output can be a np.float64 (since the results can be much larger).

This also adds type annotations to existing functions, and some new tests.

xref: HEXRD/hexrdgui#2030

Affected Workflows

All

[codecov]

Codecov Report

❌ Patch coverage is 95.45455% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.26%. Comparing base (86cf1c7) to head (e2fe8d9).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
hexrd/core/imageseries/stats.py	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #949      +/-   ##
==========================================
+ Coverage   73.22%   73.26%   +0.03%     
==========================================
  Files         146      146              
  Lines       22672    22711      +39     
==========================================
+ Hits        16602    16639      +37     
- Misses       6070     6072       +2     

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