Implement generation of ASDF inventory from headers and WCS

[Cadair]

These changes are to facilitate the automatic generation of the asdf files inside the data centre.

TODO:

• Update the asdf maker test datasets + all asdf files to use spec 214 / new asdf schema.
• tests, oh so many tests.

[codecov]

Codecov Report

Merging #76 into master will increase coverage by 0.13%.
The diff coverage is 97.34%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
+ Coverage   94.07%   94.21%   +0.13%     
==========================================
  Files          29       28       -1     
  Lines        1265     1330      +65     
==========================================
+ Hits         1190     1253      +63     
- Misses         75       77       +2     

Impacted Files	Coverage Δ
dkist/dataset/dataset.py	95.69% <83.33%> (-0.94%)	⬇️
dkist/io/asdf/generator/transforms.py	97.19% <86.66%> (-1.04%)	⬇️
dkist/io/asdf/generator/generator.py	96.49% <100.00%> (+1.66%)	⬆️
dkist/io/asdf/generator/helpers.py	100.00% <100.00%> (ø)

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