Adds a warning for iris.coord_categorisation.add_season_membership that this is not saveable as a boolean coordinate.

[HGWright]

🚀 Pull Request

Description

This addresses #5909, by raising a warning that since this adds a boolean, the result will not be saveable to a Netcdf file since it contains a boolean coordinate.

Note for reviewer:
I was not 100% sure on the type of error to raise either iris.warnings.IrisSaveWarning or iris.warnings.IrisCfSaveWarning since this a problem with Netcdf files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.80%. Comparing base (220b860) to head (bb9d1ed).
⚠️ Report is 93 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6305   +/-   ##
=======================================
  Coverage   89.80%   89.80%           
=======================================
  Files          90       90           
  Lines       23752    23755    +3     
  Branches     4418     4418           
=======================================
+ Hits        21331    21334    +3     
  Misses       1672     1672           
  Partials      749      749           

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[trexfeathers]

Thanks @HGWright; I think we can increase this warning's value by widening its scope.