Invariant to validate if a collapse will break multimesh representation

[mtao]

MultiMesh assumes that if mesh P has a child mesh C (which is a k-simplicial complex) , then each k-simplex of P always maps to a single simplex C. Collapse can break this assumption, so we need an invariant that broadcasts a precondition to all nodes checking that children maps will still be representable.

[mtao]

Looks like this is now implemented for TRI -> edge.