Segfault when reinitializing mesh connectivity

[Tormod Landet]

Following the discussion in pull request #291 the following minimal code was discovered that triggers a segfault (or an assert when that pull request is merged):

from dolfin import *
mesh = UnitSquareMesh(4, 4)
mesh.clean()
mesh.init(2, 1)

In serial the code works, in parallel it dereferences a null pointer / causes an assert in MeshEntity.entities()

[Prof Garth Wells]

We need to clean up local vs distributed mesh data, be clearer on what 'clean' does, etc.