`distributor._obj_neighborhood` attribute error in serial

[rhodrin]

Take for example the following script:

from devito import Grid

grid = Grid(shape=(8, 8))
print(grid.distributor._obj_neighborhood)

This results in AttributeError: 'mpi4py.MPI.Comm' object has no attribute 'Shift' when run in serial. This is probably not the behavior we want.

[georgebisbas]

This can be reproduced when serial but with DEVITO_MPI=0. If DEVITO_MPI=1 and mpirun -n 1 results in nb.