Consolidate angle modules

[nstarman]

A new sub-module of coordinates that is a consolidation of all the various angle stuff.
IMO this should be a public module, but I could go either way! Everything can just be imported up to astropy/coordinates.

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[nstarman]

The angle_utilities should probably have a deprecation period...

[mhvk]

I think it makes a lot of sense to consolidate this. Just a few comments, of my typical don't-change-more-than-strictly-needed-for-renaming type.

[pllim]

This should wait. I am trying to remove some deprecated stuff in another PR. There will be conflicts.

[nstarman]

Hmm. Regenerating lex and parse tab broke many things.

[nstarman]

@pllim @mhvk @eerovaher, I'm having trouble tracking which angles reference in RTD is making it fail. Additional eyes appreciated!

[eerovaher]

RTD log contains the line <unknown>:80: WARNING: py:obj reference target not found: angles, so we should grep for `angles`.
main:

$ git grep '`angles`'
astropy/modeling/rotations.py:        axis of rotation and matching the order in ``angles``.
docs/coordinates/formatting.rst::meth:`~astropy.coordinates.Angle.to_string` method (see :doc:`angles` for

This pull request:

$ git grep '`angles`'
astropy/modeling/rotations.py:        axis of rotation and matching the order in ``angles``.
docs/changes/coordinates/15220.api.rst:Angle-related classes and functions have been consolidated into the module `angles`.
docs/coordinates/formatting.rst::meth:`~astropy.coordinates.Angle.to_string` method (see :doc:`angles` for

It looks like the change log entry is causing the problem.

[nstarman]

It looks like the change log entry is causing the problem.

But how? I use double backticks so it shouldn't be linking.
Shoot. I didn't push that! I have that in my local, but not pushed to origin and then I was tearing my hair out looking at the RTD log. Thanks @eerovaher.

[nstarman]

Failures are unrelated.

[eerovaher]

We have enough code dealing with angles that creating a separate directory is a good idea, but I have a few questions and remarks about some details.

[eerovaher]

If the users are supposed to import everything from astropy.coordinates then I don't see why we would even want to mention angles, let alone angles.utils or angles.errors. We should just state that there has been some refactoring that might break some imports from some modules within astropy.coordinates, but everything that could be imported from astropy.coordinates itself still can be, and what cannot be imported from astropy.coordinates is and has been considered private.

[nstarman]

Ok, you're leaning towards "private" module (from opening comment). Happy to do that as well.

[mhvk]

Agreed with @eerovaher - let's stick with the current astropy scheme where users are supposed to import only from submodules, not below. Arguably, this is just a refactoring that does not even need a changelog entry. But I do think it is a good idea to have one anyway, if we phrase it along the lines @eerovaher suggested. It is also fine to be friendly and note that if people use stuff that is not exposed on top, they should feel welcome to submit a feature request to make it public -- since then obviously it is useful!

[nstarman]

Done.

[mhvk]

Looks good; agree with @eerovaher (and you) to import from astropy.coordinates wherever it is possible to do so. Otherwise, just nits.

[nstarman]

Resolved! It just required a rebase.

[pllim]

Linkcheck failure unrelated.

[eerovaher]

It looks like the only reason this can't be merged is that there are merge conflicts.

[nstarman]

@eerovaher, rebased!