New splitting behavior

 – Get rid of specific vtkSMPxxxAlgorithm
 – Use vtkSMPAlgorithm as an interface to specify that the algorithm has splitting capabilities
 — Implement splitting by overriding 'void SplitData(...)'
 — Enable splitting by calling SplitDatasetOn() on the filter before an update of the pipeline

 Modification of TestSMPMesh to test the new behavior
Merge branch 'experimental'