No mapping for federation option

[djogopatrao]

currently, it's mandatory to link classes to endpoints/System instances on the federation ontology. however, we could just assume that all classes are present in all endpoints; given that results from all endpoints will be unioned, querying an endpoint for a class it doesn't implements will hinder performance but ultimately won't change results. but it would turn easier to use the system at first.

keep the option of using the explicit link between endpoint and classes, though.