The current Python annotator can be compiled as single threaded or multi-threaded.
Many complex Python packages (e.g., NumPy) cannot run on a multi-threaded Python.
UIMA recognizes some annotators need to be single-threaded through a flag on its descriptor.
This flag should be honoured for the Python annotator, using a single-threaded or multi-threaded Python interpreter depending on the flag.
Depends on #8 .