The third party module vtkVPIC can be build with or without
MPI. Rather than building a MPI and non MPI version we still
need the ability to switch between the two implementations. This
can use done by move the reader into a separate module and
providing VTK_VPIC_USE_MPI. This break with rule of not having
a module change it behaviour based on compile options but unless
the third party library is refactored we will have to live with this.

Change-Id: Ia949e1860272ad5ddb7b9b4cb9eb8d88305a5783