Expose phtg reader and writer
@sebastien.jourdain @demarle @patchett2002 @Fasel
Expose the phtg reader and writer to ParaView. This enables htg to phtg or phtg to htg IO. When htg's are saved as MultiBlockDataSets, vtm to vtm reading and writing works, but vtm to htg/phtg and vice versa is not working with the error:
vtkDemandDrivenPipeline:813 ERR| vtkPVCompositeDataPipeline (0x7fc872c90e40): Input for connection index 0 on input port index 0 for algorithm vtkXMLPMultiBlockDataWriter(0x7fc872c8f400) is of type vtkHyperTreeGrid, but a vtkMultiBlockDataSet is required.
This error has occurred since vtkHyperTreeGrid changed to a vtkDataObject.