Support usage of topConceptOf without hasTopConcept

[acka47]

From skohub-io/skohub-pages#14 :

Background: I just had some students trying to built their SKOS vocab with SkoHub and it did not work because skos:hasTopConcept was missing – though the inverse skos:topConceptOf is present. The was also validated as correct by https://skos-play.sparna.fr/skos-testing-tool/ . S0 they did a good job but were frustrated by SkoHub not building the vocab and not giving any error message as of why it would not build.
...
So we should (...) think about making it sufficient to use one of two inverse properties skos:topConceptOf or skos:hasTopConcept. (I think we already do this with skos:broader/skos:barrower, dont't we?)

[acka47]

This is already supported but the validation is done before this enrichment of inverse properties and, thus, the process is stopped before inverse properties are added.