Allow data model object class to be a class that is not derived from DataObject

[kbarbounakis]

A data model object class e.g.

import {DataObject, EdmMapping} from '@themost/data';

@EdmMapping.entityType('User');
class User extends DataObject {
    ...
}

always extends DataObject class.
It will be useful to use a class which is not derived from DataObject class in order to unify development for both client and server applications.