Skip some operations in interior parents computation

[leonardo-mutti-aks]

Given that an interior parent p of an element el must satisfy p.dim() == el.dim() + 1,
we can skip some computations in MeshBase::detect_interior_parents():

1. do not check the interior parents of elements el such that there is no p with p.dim() == el.dim() + 1,
2. do not fill the node-to-elem map of candidate interior parents p if there is no el with el.dim() == p.dim() - 1.

In models with many elements, especially building the node-to-elem map can be a bottleneck,
so optimization 2 may help easing the computational effort.

[leonardo-mutti-aks]

FYI @jwpeterson .

[roystgnr]

Given that an interior parent p of an element el must satisfy p.dim() == el.dim() + 1,

So ... ex falso?

It's not used as often as for side elements, but we set the interior_parent() to the 3D element when building its 1D edges.

I'd like to see a test case that this speeds up, though; maybe there's some other way to accomplish that.