[DRAFT] relax consistency guarantees.

In order to support multi-domain spaces we relaxed the consistency guarantees between leaf-nodes and the corresponding finiteElement.

If node.size() == 0 accessing finiteElement is undefined behaviour and thus we skip nodes with size() == 0.

Currently this change only impacts the semantics, but leaves the interface unchanged. We should consider introducing a convenience function on the leaf-nodes that make the current now state explicit. Different possible names got proposed:

• active
• valid
• hasFiniteElement
• empty