Issue/blend negative values

[peterdudfield]

Pull Request

Description

Make sure there are no negative values when blendiing

How Has This Been Tested?

TDD: wrote failing test then fixed it

• [ ] Yes

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[bot]]

Codecov Report

Merging #165 (ce53e4f) into main (8ae1122) will increase coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #165      +/-   ##
==========================================
+ Coverage   94.84%   94.85%   +0.01%     
==========================================
  Files          23       23              
  Lines        1455     1458       +3     
==========================================
+ Hits         1380     1383       +3     
  Misses         75       75              

Impacted Files	Coverage Δ
nowcasting_datamodel/__init__.py	100.00% <100.00%> (ø)
nowcasting_datamodel/read/blend.py	91.79% <100.00%> (+0.18%)	:arrow_up:

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