no range restriction on SmallMoleculeReference.entityFeature property

[IgorRodchenkov]

Unlike SmallMolecule.feature and .notFeature, SmallMoleculeReference.entityFeature biopax property has no range restriction defined in the BioPAX L3 OWL. It probably should have the same feature restriction as for SmallMolecule, i.e., range: only BindingFeature.

Also, we probably should have modelled a different binding feature for small molecules and sequences to be clearer (we tended to pressure ourselves to reduce class number).