Added TypedCollection base class

[tclose]

• Permit Optional content types for TypedSet and TypedDirectory for content types that can appear (and will be listed in the contents) but aren't required

[codecov]

Codecov Report

Attention: Patch coverage is 85.49618% with 19 lines in your changes missing coverage. Please review.

Project coverage is 92.22%. Comparing base (f5bf5e6) to head (233d079).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
fileformats/core/mock.py	56.66%	11 Missing and 2 partials ⚠️
fileformats/core/utils.py	63.63%	2 Missing and 2 partials ⚠️
fileformats/core/collection.py	98.07%	1 Missing ⚠️
fileformats/generic/directory.py	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   92.25%   92.22%   -0.03%     
==========================================
  Files          45       47       +2     
  Lines        5255     5313      +58     
  Branches      597      612      +15     
==========================================
+ Hits         4848     4900      +52     
- Misses        301      304       +3     
- Partials      106      109       +3     

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