Add derivation of sea ice extent

[sloosvel]

Description

This PR allows to compute the sea ice extent as a derived variable in order to reduce the amount of saved data during the preprocessing
Closes #1693

Link to documentation:

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[codecov]

Codecov Report

Merging #1695 (439759f) into main (3149d07) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1695      +/-   ##
==========================================
+ Coverage   91.16%   91.18%   +0.02%     
==========================================
  Files         202      203       +1     
  Lines       10882    10908      +26     
==========================================
+ Hits         9921     9947      +26     
  Misses        961      961              

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_derive/siextent.py	100.00% <100.00%> (ø)

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[sloosvel]

Hi @dhohn , this is now ready to be reviewed. Do you think you could test the changes?

[dhohn]

Ive made a couple plots which look correct. I also tried a model (ACCESS) that provides both siconc and siconca, which also works.

[sloosvel]

Thanks @dhohn !

@ESMValGroup/technical-lead-development-team anyone with time to merge? I can do it if there are no further comments, but since we agreed ond having the merger be different than the reviewer I thought I would ask.

[bouweandela]

anyone with time to merge?

I would have been happy to do it. Maybe you could wait a bit longer next time, to give people an opportunity to respond to your question?

[sloosvel]

ups, will do! sorry