Add mtime_cached_property for convenient caching of properties until/unless the mtimes of the files change

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[codecov]

Codecov Report

Attention: Patch coverage is 92.98246% with 4 lines in your changes missing coverage. Please review.

Project coverage is 92.47%. Comparing base (a9a3715) to head (53e50d3).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
fileformats/core/fileset.py	85.71%	2 Missing ⚠️
fileformats/core/utils.py	94.28%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #72      +/-   ##
==========================================
+ Coverage   92.46%   92.47%   +0.01%     
==========================================
  Files          43       43              
  Lines        5041     5076      +35     
  Branches      553      559       +6     
==========================================
+ Hits         4661     4694      +33     
- Misses        286      287       +1     
- Partials       94       95       +1     

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