This was done in order to:

 - Enforce input types on representations.

 - Allow multiple inputs to representations.

 - Clean up logic around updating representations.

Other small changes that were necessary:

 - Current representation subclasses now override SetupInputConnections()
   instead of SetInputConnection().

 - vtkPassInputTypeAlgorithm (which is the superclass of
   vtkDataRepresentation) now does not error when there are no inputs.

 - vtkConvertSelection static functions now shallow copy inputs
   instead of directly passing arguments as inputs to the filter.

 - Keeping the PROP property in vtkSurfaceRepresentation's selection
   caused a nasty reference loop. This is now removed.