Fix MAE. Remove NMAE. Fix tests that were expecting NMAE!

[JackKelly]

Pull Request

Description

• Fix the calculation for MAE (it was calculating MSE :slightly_smiling_face: )
• Remove NMAE (because, as far as I can tell, this code has no way of knowing how to normalize the inputs! So I'd suggest that the calling code is responsible for normalizing the data. Because only the normalizing code knows how best to normalize the data!)
• Modify the tests that were expected NMAE!

How Has This Been Tested?

• [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

Merging #9 (c8ffd49) into jack/months (dbb614d) will decrease coverage by 0.03%. The diff coverage is 100.00%.

:exclamation: Current head c8ffd49 differs from pull request most recent head 5e63e69. Consider uploading reports for the commit 5e63e69 to get more accurate results

@@               Coverage Diff               @@
##           jack/months       #9      +/-   ##
===============================================
- Coverage        96.00%   95.97%   -0.03%     
===============================================
  Files                5        5              
  Lines              150      149       -1     
===============================================
- Hits               144      143       -1     
  Misses               6        6              

Impacted Files	Coverage Δ
ocf_ml_metrics/metrics/errors.py	92.95% <100.00%> (-0.10%)	:arrow_down:

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