add ramp rate

[dfulu]

Pull Request

Description

Add ramp rate metric.

Note this pull request can't easily be tested until after the Passiv data changes are added, so is left as a draft for now.

Notes:

• Since GSPs have very different capacities, we calculate ramp rate on the normalised truth and prediction values
• The ramp rate calculated is between the 60- and 120-minute horizons
• Ramp rate is expressed in units of: Change in percentage of GSP capacity per hour
• The metric added is the MAE of the true and predicted ramp rates
• Calculated on the model's prediction of the median value

Fixes #

How Has This Been Tested?

Will be tested locally, after

Checklist:

• My code follows OCF's coding style guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• 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]

Codecov Report

Merging #86 (afae5d2) into main (2dd1cc9) will decrease coverage by 0.25%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
- Coverage   49.87%   49.63%   -0.25%     
==========================================
  Files          23       23              
  Lines        1644     1652       +8     
==========================================
  Hits          820      820              
- Misses        824      832       +8     

Files	Coverage Δ
pvnet/models/base_model.py	32.11% <0.00%> (-1.23%)	⬇️

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