feat: More datatype support

[icweaver]

Adds support for structured (nested) datatypes, and support for ascii and ucs (also outlined in that link), which some newer Roman data files seem to use. This required changing how the datatype field is parsed, so I thought it would be better to have this as a separate PR instead of cramming it into #25.

To add this support, the new AsciiDatatype and Ucs4Datatype types were introduced. These are spiritual variants of Datatype, but since that one is an enum, it seemed like it would be more trouble than it's worth to get them to all subtype a common abstract type.

This PR also includes a grip of tests using the reference files from asdf-standard. I just included a copy of the most recent ones (v1.6.0) in the test directory here for simplicity. On a related note, should the asdf spec version printed here be updated to reflect this?

Could be a good first PR to generalize things to dynamically load different versions directly from that repo instead. Another good one might be to start refactoring src/ASDF.jl now that it is starting to get a little long

Try this PR

1. Install Julia: https://julialang.org/downloads/

2. Create a new environment:

   > julia
   
   julia> # Press ] to enter pkg mode
   
   pkg> activate my-env

3. Install this branch:

   (my-env) pkg> add ASDF#datatype

4. Load a sample asdf file:

   (my-env) pkg> # Press backspace to return to Julia REPL
   
   julia> using ASDF
   
   julia> load("path to your asdf file")

   Sample asdf files are available here.

   Documentation preview: https://juliaastro.org/ASDF.jl/previews/PR32/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (3d5ef0c) to head (bdc6ae7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
+ Coverage   99.75%   99.79%   +0.03%     
==========================================
  Files           1        1              
  Lines         413      479      +66     
==========================================
+ Hits          412      478      +66     
  Misses          1        1              

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[cgarling]

Thanks for your work on this Ian, I'm really happy to see it! I have asdf files made with romanisim that I made for some roman cycle 1 proposals that might be useful for checking against but IDK if/when I will be able to do so myself. If you'd like the files to check out yourself let me know and I can host them somewhere for you.

[icweaver]

Thanks Chris, for sure! Happy to check out any sample files you have.

[icweaver]

Update: things look to be working so far with the test files I've been sent from folks. Is there anything else that people would like to see here?

[icweaver]

Will probably give this another week and then merge for the v2 release unless I hear back from other folks

[icweaver]

Alrighty, looks like we have the remaining bits tracked now (#13), so will go ahead and merge here to move the focus over there