fix wrong R2's in plot_cv due to not subsetting training data

[mastoffel]

fixes #254

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.45%. Comparing base (e6f1931) to head (4dd28c4).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #264   +/-   ##
=======================================
  Coverage   92.45%   92.45%           
=======================================
  Files          62       62           
  Lines        3617     3618    +1     
=======================================
+ Hits         3344     3345    +1     
  Misses        273      273           

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Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
autoemulate
plotting.py
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