add function to sum up gsp values

[peterdudfield]

Pull Request

Description

Function to read all gsp values and sum them up

Helps with https://github.com/openclimatefix/uk-analysis-dashboard/issues/29

How Has This Been Tested?

Added a test

• [x] Yes

Checklist:

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

[codecov[bot]]

Codecov Report

Merging #203 (7130911) into main (2fe82ef) will increase coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
+ Coverage   94.83%   94.89%   +0.05%     
==========================================
  Files          27       27              
  Lines        1627     1644      +17     
==========================================
+ Hits         1543     1560      +17     
  Misses         84       84              

Impacted Files	Coverage Δ
nowcasting_datamodel/__init__.py	100.00% <100.00%> (ø)
nowcasting_datamodel/read/read_gsp.py	100.00% <100.00%> (ø)

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