add 20% restrict on adjust value

[peterdudfield]

Pull Request

Description

Update the adjust mw value to

• be 0 if the forecast value is 0
• restrict to 20% of forecast_value

How Has This Been Tested?

Added tests

• [x] Yes

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

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (4da59aa) 95.34% compared to head (50b8783) 95.31%. Report is 1 commits behind head on main.

Files	Patch %	Lines
nowcasting_datamodel/save/adjust.py	92.30%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #261 +/- ## ========================================== - Coverage 95.34% 95.31% -0.04% ========================================== Files 27 27 Lines 1761 1770 +9 ========================================== + Hits 1679 1687 +8 - Misses 82 83 +1 ```

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