Allow fix_file to return Cube and CubeList objects

[schlunma]

Description

Currently, if a file cannot be properly read with Iris, we use fix_file to create a copy of that file and modify it using netCDF4.Dataset (example). This is very inefficient and slow.

A much better way to deal with this is to read the file with ncdata or xarray and then use ncdata to convert that object to an Iris object. However, for this to work, we need to allow fix_file to return Iris objects (instead of paths) and load to read Iris objects (instead of paths). This PR does that.

It also provides a convenience function dataset_to_iris that allows to convert an xarray or ncdata object to an Iris object very similar to our load function.

Closes #2129
Related to #674

Link to documentation:

• https://esmvaltool--2579.org.readthedocs.build/projects/ESMValCore/en/2579/develop/fixing_data.html#create-the-fix
• https://esmvaltool--2579.org.readthedocs.build/projects/ESMValCore/en/2579/quickstart/configure.html#filter-preprocessor-warnings
• https://esmvaltool--2579.org.readthedocs.build/projects/ESMValCore/en/2579/recipe/preprocessor.html#dataset-specific-fixes
• https://esmvaltool--2579.org.readthedocs.build/projects/ESMValCore/en/2579/api/esmvalcore.preprocessor.html#esmvalcore.preprocessor.fix_file

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.96%. Comparing base (e09e396) to head (863b0f1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2579      +/-   ##
==========================================
+ Coverage   94.93%   94.96%   +0.02%     
==========================================
  Files         252      252              
  Lines       14620    14666      +46     
==========================================
+ Hits        13880    13927      +47     
+ Misses        740      739       -1     

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