Fix extract_shape when fx vars are present

[sloosvel]

Description

This pull request adds support to handle fx variables without crashing in the extract_shape preprocessor. It removes them before the shape extraction, and re-adds them after the data has been masked.

Closes #1394

Link to documentation:

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[codecov]

Codecov Report

Merging #1403 (a9bee94) into main (e4d61ad) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1403      +/-   ##
==========================================
+ Coverage   88.61%   88.62%   +0.01%     
==========================================
  Files         195      195              
  Lines        9867     9876       +9     
==========================================
+ Hits         8744     8753       +9     
  Misses       1123     1123              

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_area.py	91.57% <100.00%> (+0.28%)	⬆️

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[valeriupredoi]

brilliant! Cheers very much @sloosvel - I failed miserably at implementing this in a timely manner, you got my back tho 😁 I tested with the toy recipe from the issue - all fine, code looks smashing too 🍺 @axel-lauer this should solve the problem with the CMUG recipe, mate