Nested field mask

[iguinn]

Added ability to provide nested field masks (e.g. in evt tier, can do field_mask=["trigger", "geds/multiplicity", "geds/quality/is_not_bb_like", "coincident"]

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 10 lines in your changes missing coverage. Please review.

Project coverage is 78.40%. Comparing base (782f566) to head (487fc1a).
Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
src/lgdo/lh5/_serializers/read/utils.py	73.68%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #116      +/-   ##
==========================================
+ Coverage   78.05%   78.40%   +0.34%     
==========================================
  Files          46       46              
  Lines        3368     3376       +8     
==========================================
+ Hits         2629     2647      +18     
+ Misses        739      729      -10     

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