Enh: Add parsing of scale/units from SVS files

[psobolewskiPhD]

This PR just uses the tifffile API to check if something is SVS and then get the SVS metadata to parse out scale (which is micron per pixel), set the units, and also put the rest of the metadata in layer.metadata.
I tested locally with a file from https://openslide.org/formats/aperio/
I also add a test that writes a TIFF with SVS-like metadata.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.74%. Comparing base (dd417b3) to head (bff3740).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #65      +/-   ##
==========================================
+ Coverage   81.87%   82.74%   +0.87%     
==========================================
  Files           8        8              
  Lines         513      539      +26     
==========================================
+ Hits          420      446      +26     
  Misses         93       93              

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