Nathaniel Starkman

@pllim I don't think putting the wrapper meta class in ``utils/collections.py`` is the best place, but there isn't obviously a better one. Should we make a new file ``utils/metaclasses.py`` or...

~Oops. I can move it from codegen.~ I called the new file ``abcs.py``. I didn't want to call it ``abc.py``, like the builtin, so the plural seemed fine.

@mhvk @adrn @pllim. What do you think the next steps are? Do we need a set of tests for this beyond the tests in Masked and Distribution? More documentation? I...

> I also like the general approach @eteq , thanks! I'm very interested in getting this done as I think this metaclass can serve as the basis to make ``Quantity``...

Ping @mhvk for a review. I think it's fully functional, just a question of next steps.

Ping @mhvk. It would be to good to get this in. Do you think this needs tests beyond the docstring examples, and the ones in Distribution and Masked? I think...

Also, looking at #13305 (which I think should be merged) I think we might want to refactor ``SkyOffSetFrame`` to use this metaclass. There are a few good reasons why: 1....

> A part that I think may be overkill is the `on_metaclass` decorator - does this really add much beyond `classmethod`? Certainly, I see no reason for those methods to...

It works with SkyOffsetFrame!

> I had quite a few comments while going over the code, but after seeing everything, perhaps a larger suggestion is more important: would it make sense to make subclasses...