Switch vtk class used for Image Reader depending if MPI is built

Change-Id: I101426bf454d435000def0f94d51aa0e20f562b1
Fix ImageReader to choose between MPI and non-mpi reader

Based on build configuration a meta reader (vtkPVImageReader)
inherit from either vtkMPIImageReader or vtkImageReader.
This allow us to point to the proper one in the XML without #ifdef
that disrupt the XSL that are used to generate the documentation.

Change-Id: I6e745d9188318345f0669148bb88a12666da2610
Merge topic '13968-disable-invalid-proxy-when-mpi-off' into next

67624ab Fix ImageReader to choose between MPI and non-mpi reader
310ee8c Switch vtk class used for Image Reader depending if MPI is built