Return already created classes from factory

[CasperWA]

If the given SOFT7EntityInstance class for a given entity has already been created from the create_entity() factory function, it should simply be returned (this is done by locating it in the generated_classes module). Most likely, one should retrieve the internal entity of an existing class and ensure it is indeed exactly equal to the given entity for the create_entity() call.

This is also true for the SOFT7 DataSource classes.