add 'tag' parsing

[CagtayFabry]

After using tag in place of $ref more often (see asdf#814) I started to miss seeing the reference (+hyperlink) in the sphinx-asdf output that gets generated from $ref

Without going into details of the sphinx-asdf code I implemented a few snippets that (hopefully) parse the tag keywords similar to $ref. Some small examples I ran looked the way I expected.
Something like this:

definitions:
  root-gap:
    description: |
      Root gap
    tag: "tag:stsci.edu:asdf/unit/quantity-1.1.0"
    wx_unit: "mm"

Could this be added to sphinx-asdf?

[eslavich]

Given that we're planning to allow wildcard patterns in the tag validator, is a link appropriate here? Or would we link only if there's no asterisk character in the tag value?

[CagtayFabry]

Given that we're planning to allow wildcard patterns in the tag validator, is a link appropriate here? Or would we link only if there's no asterisk character in the tag value?

The glob asterisk sure breaks the link here so imo it seems reasonable to remove it in this case.
I did not want to fiddle with what _create_reference actually does, just get rid of the This node has no type definition (unrestricted) line when using the tag validator 😉

[CagtayFabry]

Given that we're planning to allow wildcard patterns in the tag validator, is a link appropriate here? Or would we link only if there's no asterisk character in the tag value?

I have removed the link in case tag is defined with any kind of asterisk usage @eslavich

[eslavich]

Okay, thanks! Sorry for the delay, I need to either locate someone with the expertise to review this or learn something about the package myself. I'll bring it up at our morning meeting.

[CagtayFabry]

Okay, thanks! Sorry for the delay, I need to either locate someone with the expertise to review this or learn something about the package myself. I'll bring it up at our morning meeting.

No hurry. I wasn't sure how much development is going on with sphinx-asdf
I am currently messing around to get to know (and adapt) the tool on my own fork for our project over at weldx but I thought some more generic asdf related things might be interesting for you as as well.

[eslavich]

Thanks, this looks good. At some point we'll want to use the asdf library to resolve these but that can wait for now. I'll add an issue so we remember to do that at some point.