Enable callbacks for loading netcdf with multiple variables

[stephenworsley]

🚀 Pull Request

Closes #6228

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.29%. Comparing base (9c06cca) to head (3c7b06a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6754   +/-   ##
=======================================
  Coverage   90.29%   90.29%           
=======================================
  Files          91       91           
  Lines       24656    24659    +3     
  Branches     4618     4619    +1     
=======================================
+ Hits        22264    22267    +3     
  Misses       1620     1620           
  Partials      772      772           

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[pp-mo]

I've suggested some ways to make the code cleaner, and (I think) easier to read.
Also some further tests.

Your earlier suggestion of adding some integration tests is also a good idea, I think.
But that can probably wait while we attend to other things, along with whatsnew content etc.

[pp-mo]

One final small tweak, and all good !