Add ability for MeshEditor to add markings for entities in the mesh

[James Hulsey]

TetGen can be configured to create markings for mesh entities in the mesh it creates. WHile using MeshEditor to create the Dolfin mesh from the TetGen mesh, it would be convenient if there were an easy way to use MeshEditor to also populate a MeshFunction where the entities could be easily marked like they are in TetGen.