Support isothes:subGroup and/or isothes:superGroup

[osma]

At which URL did you encounter the problem?

http://skosmos.dev.finto.fi/unesco/en/groups

What steps will reproduce the problem?

1. Use 2015 release of UNESCO Thesaurus (or old version of Metatietosanasto)
2. Look at the group index

What is the expected output? What do you see instead?

Expected to see the top level groups only (in UNESCO Thesaurus there are 7 top level groups). Instead seeing all groups with very shallow hierarchy.

As we implemented support for hierarchical groups (#186) we decided to focus on groups/collections nested using skos:member only (see #276). But now UNESCO Thesaurus has switched to using isothes:superGroup and isothes:subGroup instead of skos:member, and we again have a situation similar to #276 where the group hierarchy is not properly shown.

We would need to support hierarchical concept groups expressed as isothes:superGroup and isothes:subGroup in addition to skos:member.