Updated venv and as a result no longer need south.  Also changed the way multiprocessing is called slightly.
Shared mem implementation now seems stable, using a ctypes array pointer to share mem.
Amended TD's okabe code for KDE and added a first network SEPP placeholder- needs a lot of sorting out.
TODO: restructure SEPP models to make an abstract class that will actually be shareable between point- and network-based implementations?