Smushes the resources in mGraph that belong to the same set in equivalenceSets,
i.e. it adds all properties to one of the resources in the equivalence set.
Optionally owl:sameAs statement are added that point from the IRIs that
no longer have properties to the one with properties. If addOwlSameAs
is false the IRIs will just disappear from the graph.
mGraph - the graph to smush
equivalenceSets - sets of equivalent resources
addOwlSameAs - whether owl:sameAs statements should be added
Returns a prefered IRI for the IRIs in a set. Typically and in the
default implementation the IRI will be one of the set. Note however that
subclass implementations may also return another IRI to be used.