feat: variable group

[tmichela]

Adds the VariableGroup class to organize related variables into components.

https://damnit--478.org.readthedocs.build/en/478/configuration_and_management/#group

[codecov]

Codecov Report

❌ Patch coverage is 95.26066% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 79.37%. Comparing base (93a2b3d) to head (37f73cd).
⚠️ Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
damnit/ctxsupport/damnit_ctx.py	95.91%	8 Missing ⚠️
damnit/backend/extract_data.py	66.66%	1 Missing ⚠️
damnit/ctxsupport/ctxrunner.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #478      +/-   ##
==========================================
+ Coverage   78.84%   79.37%   +0.52%     
==========================================
  Files          37       37              
  Lines        6562     6763     +201     
==========================================
+ Hits         5174     5368     +194     
- Misses       1388     1395       +7     

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[tmichela]

@takluyver I think the last change adresses what we discussed yesterday.

[tmichela]

I'll merge in a couple days, if there's no new comments.

[takluyver]

Would it be simpler to make tags always a tuple, using () as the default instead of None?

[tmichela]

I went for the same default as Variable, for consistency.

[tmichela]

closing in favour of #532