Fix, remove, or extract `BoundaryPatch::getPatchBoundaryVertices()` and `BoundaryPatch::getPatchBoundaryEdges()`

Both methods rely on the consistency of IndexSet::subIndex() and ReferenceElement::subEntity() which is not guaranteed by the grid interface. IMHO we should either fix or remove these methods. At least we should extract them from the class and make them free functions and possibly add a list of grids where this assumption is true.