mesh.init() computes all connectivity and should almost never be
called, at least not from inside DOLFIN.
Instead, the proper init function for the connectivity that is
actually used should be called, like mesh.init(1, 2) or similar.
Note that calling init functions is not needed when using iterators
since the connectivity is then automatically initialized.
The mesh that comes out of refine() is a clean mesh that only has
cell-vertex connectivity so it's not strange that one needs to call
some init function.
