Fixed problem in single ref. Added concept of model enhancement

User's Profile and Preference enhancement models added and tested with
population (read op only from data api routes);
By design, we will not support post/put or delete routes of one-to-one
ref docs, it is designed to be manipulated through exposed logic routes.
The one-to-one ref model acts as a parent model's enhancement.
Note that if a model appears in another model's schema as a one-to-one
ref, it is marked as a enhancement model (in the def meta) and thus
won't appear in the resource list as stand alone nor refed fields.
Enhanced parent model is in charge of providing logic apis for
manipulating the data parts.
Mutex should be applied and manipulation of the enhanced data parts
should be treated as parent's data in all situations.