Allow access of the filenames from the dataset object

[Cadair]

No description provided.

[codecov]

Codecov Report

Merging #56 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   97.42%   97.45%   +0.02%     
==========================================
  Files          21       21              
  Lines        1050     1061      +11     
==========================================
+ Hits         1023     1034      +11     
  Misses         27       27

Impacted Files	Coverage Δ
dkist/dataset/dataset.py	96.66% <100%> (+0.07%)	⬆️
dkist/io/fits.py	100% <100%> (ø)	⬆️
dkist/io/reference_collections.py	100% <100%> (ø)	⬆️

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