Allow property overriding

[ledsoft]

As a user, I want to be able to override property mapping. The main use case after implementing #16 would be preventing term mapping conflicts in context, e.g., when two classes have a field with the same name but different mapped property, it is not possible to serialize them at the same time because a term mapping conflict would occur in the context. Property overriding would solve this.

Note that this has to work both for serialization and deserialization.