Extract gwcs extension to plugin library

[embray]

I never intended the gwcs support to be included in the main libasdf, but developed it in the libasdf source tree for convenience. I think we need to create a new repository libasdf-gwcs to contain the extension code. @braingram do you think you could create such a repository to have ready?

I think for the first release, at least the alpha, of libasdf I will still include the gwcs support for convenience, but split it out later. Requires #39

[braingram]

Happy to make a repository. Let me know what you'd like it called and if there's any other setup I need to do. Also are you good with the same permissions, etc as this repo?

[embray]

Let's just call it libasdf-gwcs I guess? Or do you think just "libasdf-wcs" is better?

[braingram]

I set this up: https://github.com/asdf-format/libasdf-gwcs
and granted you permissions.

Let me know if that works and if you're able to change the things you need to for setting up that repository.

I'm happy to rename it as you see fit.

[embray]

Thanks! LGTM.