Add option to fit data to a time varying GEV

[stellema]

This PR closes #45.

[codecov-commenter]

Codecov Report

Merging #52 (9f1a958) into non_stationary_gev (89fdaa0) will increase coverage by 9.85%.
The diff coverage is 46.80%.

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@@                  Coverage Diff                   @@
##           non_stationary_gev      #52      +/-   ##
======================================================
+ Coverage               22.67%   32.52%   +9.85%     
======================================================
  Files                      18       18              
  Lines                    1632     1691      +59     
======================================================
+ Hits                      370      550     +180     
+ Misses                   1262     1141     -121     

Files	Coverage Δ
unseen/tests/conftest.py	95.89% <100.00%> (+2.27%)	⬆️
unseen/moments.py	10.41% <50.00%> (+10.41%)	⬆️
unseen/stability.py	15.18% <38.88%> (+15.18%)	⬆️
unseen/general_utils.py	13.37% <20.83%> (+13.37%)	⬆️

... and 7 files with indirect coverage changes

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