added get_valid_midpoints to gridded forecast

[wsavran]

No description provided.

[codecov-commenter]

Codecov Report

Merging #137 (9d0cb07) into master (4a789d9) will decrease coverage by 0.11%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #137      +/-   ##
==========================================
- Coverage   56.97%   56.86%   -0.12%     
==========================================
  Files          19       19              
  Lines        3147     3155       +8     
  Branches      455      457       +2     
==========================================
+ Hits         1793     1794       +1     
- Misses       1243     1250       +7     
  Partials      111      111              

Impacted Files	Coverage Δ
csep/core/forecasts.py	54.24% <12.50%> (-1.27%)	⬇️
csep/core/regions.py	53.65% <100.00%> (ø)

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