Feature/poisson source arrivals

Hi Arne,

This pull request implements a Poisson process arrival queue for macroscopic traffic flow sources. The Poisson process here gives a reasonable random approximation of vehicle queueing rather than a purely linear model with identical arrival times. I have implemented this for my own project using Movsim and thought you may also be interested in as part of the stable release. 

Let me know your thoughts.

Regards,
James