⚡️ Allow FillGapsAR to perform prediction on data used for fitting.

[BaptisteDE]

Mainly for Prophet.
Added tests for STL.
Running them takes forever

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.30769% with 4 lines in your changes missing coverage. Please review.

Project coverage is 87.80%. Comparing base (5e1eddf) to head (0c0ba61).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
tide/processing.py	92.30%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   87.54%   87.80%   +0.25%     
==========================================
  Files          12       12              
  Lines        1333     1345      +12     
  Branches      185      188       +3     
==========================================
+ Hits         1167     1181      +14     
+ Misses        102      101       -1     
+ Partials       64       63       -1     

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