Fix error is VTK_MPI flags are not set.

When using external VTK,  VTK_MPI_* flags are not imported. Just pick decent
defaults, in that case.

Change-Id: I4d5cf6a8ab9bc64ee25e6fbfd5a0ef34dbf8d4cd
Moving optional dependencies to VTKModules.cmake.

Adding optional depedencies to VTKModules.cmake is a better way to keep track of
such dependencies.

Change-Id: Id87cdfd2033bcda3350c4537158b79edd086e7b5
Use vtkWrappingPython module correctly.

vtkWrappingPython is now a true VTK module (which wasn't the case the past).
Fix the code to use this new module. Makes it work with external VTK.

Change-Id: Ia04c5bbb79b0e2b7616281f6a1040027b3a9c6d6
Cleanup compiled Python modules when using external VTK.

Cleaned up the code that compiles the Python modules for ParaView when using
external VTK. Python support for external VTK is still under works, but this
overcomes the *.py files issues.

Change-Id: Ie72121033d9ee540ca2bad24a6d634d2abf5109f
Changes to make ParaView use external VTK.

This adds support to use externally built VTK with ParaView. Python wrapping is
not yet supported with external VTK, but in other cases, one can simply point to
a prebuilt VTK and use it.

Change-Id: Ifcc88eb2d73a824c117af047eae1b7812c69dd0b
Merge topic 'use_external_vtk' into next

cb3e1e9 Changes to make ParaView use external VTK.
1171fff Cleanup compiled Python modules when using external VTK.
da197e7 Use vtkWrappingPython module correctly.
5dc2c8c Moving optional dependencies to VTKModules.cmake.
67090cb Fix error is VTK_MPI flags are not set.