Issues #283 #373 Removed AbstractEntity

* Abstract entity has been removed. Refactored all entities affected
* Fixed Issue 373, checking now type of entity
* Tests for Issue 373 resolution
* Cleaned Interfaces of Variant. Removed ResolverInterfaces and default implementation. Set
the logic inside the Variant Object (Easier for Interfaces and more easy for the final users)
Merge pull request #422 from elcodi/feature/removed-abstract-entity

Issues #283 #373 Removed AbstractEntity