Add doc strings to constants and add SOFA names

[david-macmahon]

Add doc strings to constants defined in src/constants.jl. Also add missing SOFA/ERFA names for constants. Even though they are somewhat cryptically named they are useful when porting code that uses them.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.75%. Comparing base (23eced0) to head (1241556).

Additional details and impacted files

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  Coverage   92.75%   92.75%           
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  Files          24       24           
  Lines        1685     1685           
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  Hits         1563     1563           
  Misses        122      122           

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[barrettp]

constants.jl is primarily for the Astrometry.jl package, not the SOFA subpackage, so there should be no reference to it in the constants.jl file. You are welcome to add those definitions to a constants file in the SOFA subpackage.

[david-macmahon]

The src/constants.jl file is included by both src/Astrometry.jl and src/sofa.jl. Shall I create a new file src/sofa_constants.jl and include that in src/sofa.jl instead of src/constants.jl? Or put only the "missing" ones in src/sofa_constants.jl and include it in src/sofa.jl in addition to src/constants.jl?

[barrettp]

At this stage, it would probably be best to make the SOFA submodule completely independent of the Astrometry module by duplicating all of the constant files and placing them in the SOFA submodule.

It would be even better to replace the wrapped SOFA.jl package with the pure Julia SOFA submodule. We should contact the owner of the SOFA.jl package to see if they would be interested in such a change.

[david-macmahon]

What do you mean by "replace the wrapped SOFA.jl package with the pure Julia SOFA submodule"? Isn't this already pure Julia? Maybe I misunderstand things. I thought Astrometry.jl is intended to be a pure Julia version of SOFA.

[barrettp]

I am suggesting that the wrapped C library of either SOFA.jl or ERFA.jl be replaced by the pure Julia SOFA submodule of Astrometry.jl, now that a pure Julia version exists. In other words, yank the SOFA submodule out of Astrometry.jl and put it in SOFA.jl (or ERFA.jl), so that the two modules are separate and distinct.

[david-macmahon]

What would be left in Astrometry.jl? I guess I always thought of Astrometry.jl as being "just" a pure Julia implementation of SOFA rather than being a user (and creator) of a pure Julia SOFA while also providing higher level functionality.

[barrettp]

Astrometry.jl is meant to be simplified, yet complete, redesign of the SOFA functions. The SOFA submodule was added to check the accuracy of the Astrometry functions, but was not intended to be the primary module. It just happens to be now because not much development has been done on the main module. This is mainly because there is no TimeScales.jl and ReferenceFrames.jl packages. In other words AstroTime.jl is incomplete in my opinion, because the reference frames are implied, but not explicit. The goal is to develop a holistic package. This is the reason why I want to keep the SOFA submodule independent of the Astrometry module.

[david-macmahon]

I think it would be great to have a standalone pure Julia implementation of the SOFA/ERFA library. I'm not sure the best way to package that, but it's probably better discussed in an issue rather than this PR. 😅

[tamasgal]

I agree that we should discuss this separately ;)