Enable using the SKOS concepts of other ontologies in "tree generation" or "broader/narrower/related node identification of SKOS vocabularies

[anganske]

SKOS vocabularies cannot be ingested into the TIB TS, if they use concepts of other vocabularies in "tree generation" or "broader/narrower/related node identification", as it is the case in http://vocab.nerc.ac.uk/collection/P07/current/
As the BITS project urgently needs this vocabulary in its ESS collection, it would be of great help if the SKOS module would be updated so that we can also use modular SKOS vocabularies.
@giraygi