Allow read_metadata kwargs to be passed to FileSet.__init__

[tclose]

Changes the signature of FileSet.init to:

• allow var kwargs, which will be interpreted as kwargs to be passed to read_metadata implementations
  • Note, metadata is the only reserved keyword, which can take explicit metadata
• fspaths are now varargs so SetOf[Png]('/path/to/one/png.png', '/path/to/another/png.png') is now valid

load, save extras hooks and the new method (which uses save to create a new fileformat object) have been moved to base FileSet class.

[codecov]

Codecov Report

Attention: Patch coverage is 74.64789% with 18 lines in your changes missing coverage. Please review.

Project coverage is 92.14%. Comparing base (45ee936) to head (48aa15e).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
fileformats/core/extras.py	51.72%	9 Missing and 5 partials ⚠️
fileformats/core/mixin.py	77.77%	1 Missing and 1 partial ⚠️
fileformats/image/raster.py	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #79      +/-   ##
==========================================
- Coverage   92.37%   92.14%   -0.23%     
==========================================
  Files          45       45              
  Lines        5219     5234      +15     
  Branches      580      586       +6     
==========================================
+ Hits         4821     4823       +2     
- Misses        301      307       +6     
- Partials       97      104       +7     

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