Preserve composite-data names in legacy readers/writers.

vtkCompositeDataWriter/vtkCompositeDataReader didn't preserve composite data
block names. Fixed that.

Change-Id: Iad7c950bab05064f617e881f09cd7ff179398673
Fix send/receive for composite datasets.

Send/Receive for composite datasets was using custom logic
which predated the fact that vtkGenericDataObject[Reader|Writer]
supports composite datasets. Fixed that. vtkCommunicator no longer needs
custom logic to ship composite datasets.

Change-Id: I94fbac0abb83069281cb3c2fa9e223b5cca63a0d
Merge topic 'fix_composite_data_names' into master

af508c83 Fix send/receive for composite datasets.
84c0666e Preserve composite-data names in legacy readers/writers.