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

Attention: Patch coverage is 84.21053% with 6 lines in your changes missing coverage. Please review.

Project coverage is 93.72%. Comparing base (0787a89) to head (77c601a).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/useq/_iter_sequence.py	66.66%	5 Missing ⚠️
src/useq/_mda_event.py	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #216      +/-   ##
==========================================
- Coverage   94.03%   93.72%   -0.31%     
==========================================
  Files          22       22              
  Lines        1492     1515      +23     
==========================================
+ Hits         1403     1420      +17     
- Misses         89       95       +6     

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