Merge pull request #2 from geraintpalmer/master

added update next event, wrote more tests and moved max_simulation_time ...
Found small bug in init of simulation when tweaking init of node
Have a class for an exit node
Have tests and arrival node class written
Have transitions out of active nodes now going to exit node as well
Fixing exit node in overall simulation - now takes total time as requireed
First draft of model running (maybe) need to add in summary statistics and data collection
Data collection written - needs testing and tests - have errors
Have added tests to have an event
Added more tests