Ensure that all processes in subgroup invoke collectives

In a distributed dataset where some processes have datasets with no points or cells, not all processes would invoke the same collective communication operations in vtkPKdTree::CreateGlobalDataArrayBounds(). This would lead to a deadlock in pvserver where processes waited for messages that never came.

This is solved by distributing the maximum number of cell and point data arrays to all processes so that they all participate in the collective operations.