list the metadomains in domains.xml, with a brief description; constrain the values of sortBy

[bansp]

Metadomains are merely convenience groupings, so that it's easier to identify the kind if function that the particular domain describes. But they deserve a short description, and a place where they can be listed, so that it's more difficult to make an encoding mistake in the value of @sortBy. So:

• add to the schema, to list metadomains at the beginning of domains.xml
• add schematron that uses the names of metadomains for constraining @sortBy
• errm, perhaps even change the attribute name now that metadomains gain flesh (nahh, no need)
• make it all display nicely in https://standards.clarin.eu/sis/views/list-domains.xq
• (perhaps) make it possible for metadomains to overlap; but let this one remain a task for the future

[bansp]

This is ready to be closed; the potential task "make it possible for metadomains to overlap" should be handled elsewhere (but it may be better to wait and see if a real need for that arises).