cleanup of repetitive code

[jgrewe]

with pr #558 I introduced even some more repetitive code. We have several instances where we have sets of methods of the following pattern:

• [ ] entityCount()
• [ ] getEntity()
• [ ] addEntity()
• [ ] hasEntity()
• [ ] removeEntity()
• [ ] entity(vector<Entity>)

which are mostly clones from each other. While this is ok for the front-facing objects It is at least ugly in the actual implementations.

[jgrewe]

has been greatly reduced with pull request #679