Loading a mesh from .xml does not init *_global attributes

[Stephan Schmidt]

if you load a mesh from a .xml file, the *_global attributes are only initialized on two or more processors.

from dolfin import *

MeshName = "3D/square_0_5"
mesh = Mesh("mesh/"+MeshName+".xml")
#mesh = UnitSquareMesh(10, 10)

mesh.init()
mesh.order()

print("local Num Entities: %d"%mesh.num_entities(2))
print("global Num Entities: %d"%mesh.num_entities_global(2))

if you run the above minimal example with any .xml in serial or mpi -np 1, the global number is zero. Only with two or more processors, the _global number will have a value.

But if you use the built-in mesh, then the *_global is always correctly initialized