Offer different strategies for model constraint violations

[SusieGoebel]

Formerly, we enforced model consistence all the time. Now, we have a slightly more relaxed approach towards model creation and could hence also think about models becoming inconsistent along their life cycle. Think of different scenarios for that (considering the current case of either list or singleton, where a singleton could be required.
Later we will also add cardinalities for both on the container level and consider consistence there (see #15 ).

So, propose:

• consistency checks for required singletons
• deletion handling that makes models consistent by cascading deletes (so that any component that becomes inconsistent by deletion of one required singleton simply dies as well)
• deletion handling that allows inconsistent models and just maintains elements and their inverses in consistent state

When we also add other cardinality constraints, models might also become inconsistent on addition

• if you think, that this can happen already now, write a comment and a test that shows the behaviour.