ENH: Use GatherV in CollectInformation()

This commit re-implements the CollectInformation() method
of the vtkPVSessionCore class to use MPI GatherV collective
operation when communicating information (vtkPVInformation)
from the satellite ranks to the root node, rank 0.

Change-Id: I757c53f5365e2f2f9f9d6bcef051cdd008151efa