allow non-standard schema for units or coordinate frames

[jvandegriff]

We should allow for 'units' schemas that are not in the restricted list kept in the HAPI spec.

And if the 'units' strings for a dataset refer to a units schema that is not listed in the HAPI spec document, there needs to be a way to indicate where to find info on the schema. Note that this should still be some kind of computer-readable schema.

So, the 'units' specification in the 'info' response should have a way to give a reference for the schema name used. A reference could be a URL or a DOI.

Since different datasets could use different 'units' schemas (or different coordinate frame schemas, once we have those), it makes sense to have the schema reference (URL or DOI) be included in the 'info' metadata, since that's where you would need to be using it. (One suggestion was that all of a server's custom schema references could go in the 'about' endpoint, but this is overly complicated - just keep the schema reference close to where it is needed. If you have multiple datasets with the same schema (i.e., repeated in many 'info' responses), that does not really hurt anything - those servers will likely have info responses filled out automatically / programmatically.