issue-120 importing skos into sosa

Firmly agree that we should keep SOSA as import-free as possible.

In particular for skos:

Some tools will import it anyway just because skos terms are used, but that is their problem.
The way skos is used in sosa, as annotations, no skos inference is needed and there isn't even  a need to understand the skos terms in order to make use of them (although some tools might do something sensible with them).

OTOH, skos is technically OWL-FULL (although arguably need not be) , so enforcing its import is just asking for trouble.

-Kerry