[cat-interop] Metadata link types vocabulary proposal

[Tom Kralidis]

Hi all: update on this activity. We've crafted an initial,
short/simple/extensible list at:

https://github.com/OSGeo/Cat-Interop/blob/master/link_types.csv

Associated ticket: https://github.com/OSGeo/Cat-Interop/issues/1

The goal here is to have consensus among content providers and
implementations so something like this can be implemented.
Eventually, the list will be versioned and titled something like "Link
Type Vocabulary" with the author being "OSGeo Catalogue
Interoperability Working Group".

Here, it would be great for deegree2, GeoNetwork, pycsw and others to
support this, and it is a great time for finally harmonizing such an
important piece of interoperability in a pragmatic manner.

This also bring s us closer to realizing a better "add to map"
workflow/use case, a core driver of this WG.