WIP - Task/refactor transformers

Bug fix: [no]
Feature addition: [no]
Backwards compatibility break: [no]
Unit test passes: [yes]
Behat scenarios passes: [yes]
Checkstyle issues: [no]*
ChangeLog updated: [no]
Documentation PR: 
Fixes the following jira:

This PR makes nested transfomers more readable, testable and extendable by introducing an EntityTransformerRegistry.
All OrmTransformer classes have been renamed to drop the 'Orm' part, which was only there for historic reasons.