Unblock global to PETSc numbering in parallel case

We currently build the correct global to PETSc numbering maps for
block matrices, but can only assemble into non-block matrices.  So as a
quick fix for assembling on vector function spaces in parallel, use
PETSc routines to build the appropriate unblocked maps.
Merge pull request #273 from OP2/parallel-vfs

Unblock global to PETSc numbering in parallel case.

We currently build the correct global to PETSc numbering maps for
block matrices, but can only assemble into non-block matrices. So as a
quick fix for assembling on vector function spaces in parallel, use
PETSc routines to build the appropriate unblocked maps.