change to VecScatterBegin/End

[Satish Balay]

The prototypes for VecScatterBegin() and VecScatterEnd() are now
changed. This change is made to have these routines consistant with
PETSc calling convention [i.e the first argument should be the
appropriate object for that functionality]. i.e:

PetscErrorCode VecScatterBegin(Vec,Vec,InsertMode,ScatterMode,VecScatter);
to
PetscErrorCode VecScatterBegin(VecScatter,Vec,Vec,InsertMode,ScatterMode);

Satish