Changed the default executive to be vtkCompositeDataPipeline.

This will enable processing of composite datasets without
having to override the default pipeline manually.

Change-Id: I1b62bca740b4d67f58818bd2b2ebf9a5ad785867
Merge topic 'composite-pipeline-as-default' into master

2ef97890 Changed the default executive to be vtkCompositeDataPipeline.