BUG: When using the Py_NewInterpreter api there was no way to tell vtkPythonCommand which interpreter to use when executing the callback.  Added a SetThreadState method so that vtkPythonCommand may call PyThreadState_Swap in order to make the interpreter for the specified threadstate the active interpreter.  See vtk-developers email from June 18, 2009 with subject 'Py_NewInterpreter and vtkPythonCallback issue'