[cat-interop] link types update

[Tom Kralidis]

All: FYI we are continuing to work away at firming this up.  Please
follow the discussion at https://github.com/OSGeo/Cat-Interop/issues/1

The current thinking looks like to mint this list at
https://github.com/opengeospatial/md-vocab when final so that
applications can start to implement from an authoritative space.

Comments, contributions and feedback are welcome and encouraged.