Fix GP model specification in s5_fit.py

[jbrande]

Handling issue #746 and further GP testing, if needed.

[jbrande]

I couldn't get george to work at first, but I think I've diagnosed that now. There was another issue in the GPModel eval(), where george GPs would end up with evaluated models of different lengths than the time axis, caused by a minor typo neglecting to pass the mask through to the kernel input in the predict() step. George is still painfully slow, though, so this might not actually help anyone, lol.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 57.69%. Comparing base (93c15c6) to head (2b9c8be).
Report is 130 commits behind head on main.

Files with missing lines	Patch %	Lines
src/eureka/S5_lightcurve_fitting/models/GPModel.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #747      +/-   ##
==========================================
+ Coverage   57.03%   57.69%   +0.66%     
==========================================
  Files         113      113              
  Lines       14362    14475     +113     
==========================================
+ Hits         8192     8352     +160     
+ Misses       6170     6123      -47     

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