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.

throws an exception, it's handled by calling vtkErrorMacro() with the error
message that is passed with the exception object.  This functionality is
especially crucial when ITK and VTK pipelines are connected: ITK throws
an exception when an error occurs.  If VTK doesn't catch this, the
application is unceremoniously dumped to the command-line.

Thanks to Ken Martin for the idea of implementing this handling directly
in vtkImageImport.

ENH: Simplified input/output port information access by making methods work with port index instead of one vtkInformationVector containing all ports.