add optional sum-of-GSP saving

[dfulu]

Pull Request

This saves the sum-of-GSPs under a different model name to the logging. This will allow us to compare the performance of the summation model to the simpler sum-of-GSPs, and help us monitor the summation model's performance more easily.

This is part of #15.

Includes:

• Optional saving of the sum-of-GSPs using the environmental variable SAVE_GSP_SUM (default: False)
• If saved the sum-of-GSPs is saved under the model name pvnet_gsp_sum
• Updated tests

Checklist:

• [x] My code follows OCF's coding style guidelines
• [x] I have performed a self-review of my own code
• [x] I have made corresponding changes to the documentation
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] I have checked my code and corrected any misspellings

[codecov[bot]]

Codecov Report

Merging #16 (bf00d72) into main (e19702c) will increase coverage by 0.34%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #16      +/-   ##
==========================================
+ Coverage   92.89%   93.23%   +0.34%     
==========================================
  Files           2        2              
  Lines         197      207      +10     
==========================================
+ Hits          183      193      +10     
  Misses         14       14              

Files	Coverage Δ
pvnet_app/app.py	93.20% <100.00%> (+0.34%)	:arrow_up:

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