ENH: Added protected virtual CreateDefaultExecutive method to allow subclasses to change the default executive created.

should be set to an empty volume.  Some filters don't honour UpdateExtent.

directly in ThreadedExecute() for non-NULL value.  inData is only valid
up to the NumberOfInputs - 1, if you check for inData[i] for an invalid
i, you're getting some value past the end of the array!  Use
this->GetInput(i) for your checks instead.  Once you've established that
a certain i is valid, you can use inData[i].

ENH: Replaced vtkInformationDataSetKey with vtkInformationDataObjectKey.  Also added vtkInformationDataObjectVectorKey and DATA_OBJECTS key instance.

ENH: Added request argument to ProcessDownstreamRequest and ProcessUpstreamRequest that is independent of input and output ports of the algorithm.