Make instantiating the quotient graph of an `AbstractDataGraph` return a `DataGraph` as a fallback.

[jack-dunham]

Currently it returns a NamedGraph (the fallback for AbstractNamedGraph) if the function quotient_graph is unspecialized. The issue with that fallback is that it makes the behavior of the QuotientView (which behaves like a data graph) incongruent with the instantiated quotient graph (which will not behave like a data graph by default).