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.

start of the UpdateCursor() call.  Without this, changes in an up-and-
running pipeline can cause segfaults if the user clicks directly on an
IPW.

     1) this makes help() work nicely on vtk classes, since __dict__ is used for introspection
     2) since __dict__ is a hash, VTK function calls no longer require a linear lookup and should be faster than before
     The __dict__ is only created for VTK classes that are actually used, so the memory overhead should be small.  Note that VTK classes and instances no longer have __members__ or __methods__ special attributes.