refactor: remove sec_cats

[mikapfl]

Pull request

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Description in a {pr}.thing.md file in the directory changelog added - see changelog/README.md for details

Description

Remove sec_cats:

• from the documentation of the data formats
• from tests
• from the formal definitions of data formats

However, we retain:

• backwards compatibility for reading: We just ignore sec_cats when loading from CSV or netcdf.
• when converting free-form DataFrames to interchange format, you still have to specify secondary categories using the sec_cat__ prefix, for backward compatibility and because it's a useful feature to guard against typos.

Note that this is a breaking change, if you e.g. read something from interchange format, then save it back to interchange format, it will remove the sec_cats entry.

Closes: #217

[mikapfl]

Unfortunately, there are a lot of places where sec_cats were used. The tests now pass, but I still think this is a medium-risk PR. Maybe we can test something from the non-AnnexI repo to see if it still works?

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.18%. Comparing base (5a5deb0) to head (3930b2b).
Report is 10 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
primap2/pm2io/_interchange_format.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #277      +/-   ##
==========================================
- Coverage   97.25%   97.18%   -0.08%     
==========================================
  Files          47       47              
  Lines        4441     4399      -42     
==========================================
- Hits         4319     4275      -44     
- Misses        122      124       +2     

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