Initial pass at a two layer refactor of FileManager

[Cadair]

This is an attempt to tidy up the API of FileManager in a way that makes it easy to apply many slices to the same underlying file manager object.

Fixes #166

[Cadair]

pre-commit.ci autofix

[codecov]

Codecov Report

Merging #176 (efd147f) into main (963b2b1) will increase coverage by 0.08%.
The diff coverage is 98.47%.

@@            Coverage Diff             @@
##             main     #176      +/-   ##
==========================================
+ Coverage   96.27%   96.35%   +0.08%     
==========================================
  Files          28       28              
  Lines        1451     1483      +32     
==========================================
+ Hits         1397     1429      +32     
  Misses         54       54              

Impacted Files	Coverage Δ
dkist/io/file_manager.py	96.38% <98.38%> (+0.86%)	⬆️
dkist/io/asdf/converters/dataset.py	89.65% <100.00%> (ø)
dkist/io/asdf/converters/file_manager.py	96.00% <100.00%> (ø)
dkist/wcs/models.py	100.00% <100.00%> (ø)

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