Add a Dataset.download method

[Cadair]

This was the easy bit. Now I have to actually make a test dataset that I can test it against (and write unit tests obv)

[codecov]

Codecov Report

Merging #57 into master will increase coverage by 0.04%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #57      +/-   ##
=========================================
+ Coverage   97.45%   97.5%   +0.04%     
=========================================
  Files          21      21              
  Lines        1061    1082      +21     
=========================================
+ Hits         1034    1055      +21     
  Misses         27      27

Impacted Files	Coverage Δ
dkist/asdf_maker/generator.py	100% <ø> (ø)	⬆️
dkist/utils/globus/endpoints.py	100% <ø> (ø)	⬆️
dkist/dataset/dataset.py	97.22% <100%> (+0.55%)	⬆️
dkist/utils/globus/__init__.py	100% <100%> (ø)	⬆️

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