Improved GP fitness to use NRMSE

[jmafoster1]

GP fitness now uses NRMSE instead of just MSE, fixed a couple of other bugs, and removed a print statement.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ PYTHON	black	36		0	1.0s
✅ PYTHON	pylint	36		0	5.79s

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.02%. Comparing base (18d0356) to head (f22af96).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #300      +/-   ##
==========================================
+ Coverage   97.01%   97.02%   +0.01%     
==========================================
  Files          29       29              
  Lines        1842     1849       +7     
==========================================
+ Hits         1787     1794       +7     
  Misses         55       55              

Files with missing lines	Coverage Δ
...stimation/genetic_programming_regression_fitter.py	98.86% <100.00%> (+0.04%)	⬆️

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[f-allian]

@jmafoster1 Thanks Michael, I'll put some time aside to review this this afternoon.

[f-allian]

@jmafoster1 Some quick fixes - see comments above.

[jmafoster1]

Thanks @f-allian, fixed them all