feat: add non_dimension_coords to MDAEvent

[tlambert03]

closes #196, allowing for additional coordinates to be attached to an MDAEvent, that are a "true axis" of the sequence. For example, in a well plate where you have selected 12 wells, you will see index = {"p": 0 } ... {"p": 11} but each position will now also have non_dimension_coords with something like: {"r": 0, "c": 1}.

[codecov]

Codecov Report

❌ Patch coverage is 68.75000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.72%. Comparing base (fec18af) to head (77c601a).
⚠️ Report is 38 commits behind head on main.

Files with missing lines	Patch %	Lines
src/useq/_iter_sequence.py	58.33%	5 Missing ⚠️

❌ Your patch check has failed because the patch coverage (68.75%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
- Coverage   94.28%   93.72%   -0.57%     
==========================================
  Files          22       22              
  Lines        1506     1515       +9     
==========================================
  Hits         1420     1420              
- Misses         86       95       +9     

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