BUG: Bug was introduced following fix for 4823. When the python programmable filter was used through the GUI, it would hang on windows. The reason being that the interpretor used by the filter did not use locking (locking was used by the interpretor in the python shell and the testing thread). However, we still don't want the filter interpretor to use the lock if it is executed through a python shell as the python shell would already have acquired the lock. This fixes the issue by making the interpretor used by the filter use locking when a lock is not already held by another instance of vtkPVPythonInterpretor. As a consequence, however, vtkPVPythonInterpretor cannot be used in different threads.