Add parallel CGNS writer
This MR contains a parallel CGNS writer. It collects pieces from all processes onto the main process, uses an appendfilter to create one dataset and uses the existing vtkCGNSWriter to write the data.
Additionally, it contains a fix for a potential memory leak of vtkCGNSWriter. When a writer object is used multiple times for different inputs, it retains a refcount on each input it has written, without releasing. This is now fixed in this MR.
Edited by Menno Deij - van Rijswijk