Removed scalar coordinates p0 and ptop prior to merge in multi_model_statistics

[axel-lauer]

This is a solution for the issue described in #1222.

Description

Variables on hybrid vertical levels (e.g. clw) have to be converted to pressure or height levels to be able to be processed by some diagnostics and to be able to calculate meaningful multi-model statistics across different models. For this conversion, some models provide an auxiliary coordinate 'p0'. Calculating the multi-model mean over such datasets converted to pressure or height levels fails because 'p0' is kept after converting the vertical levels but (as expected) not identical in all models. This problem is solved by deleting the scalar auxiliary coordinates p0 and ptop (if present) after converting to pressure or altitude levels.

Closes #1222

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[schlunma]

I like it!

In my opinion we could even go further and completely ignore scalar coordinates altogether (i.e., removing them before the merge and re-adding them later if necessary). I've had so many troubles with them (e.g., SciTools/iris#3584, just today!)...

However this might be a topic for another pull request. As far as I understood there might be also better support by iris in the future SciTools/iris#4446).

[codecov]

Codecov Report

Merging #1471 (e10578b) into main (c788735) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1471   +/-   ##
=======================================
  Coverage   89.84%   89.84%           
=======================================
  Files         196      196           
  Lines       10473    10477    +4     
=======================================
+ Hits         9409     9413    +4     
  Misses       1064     1064           

Impacted Files	Coverage Δ
esmvalcore/cmor/_fixes/shared.py	100.00% <ø> (ø)
esmvalcore/preprocessor/_multimodel.py	95.50% <100.00%> (+0.10%)	⬆️

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[schlunma]

🎉

(Failing tests are not related to this PR)

[axel-lauer]

Just tested this again and it works nicely. This can be merged now.

[schlunma]

Great, thanks Axel! Just merged the main into this to make the tests pass, will merge afterwards.