Figure out where to have calculate_travel_time

[SeanMcOwen]

def calculate_travel_time(self, zone1, zone2):
    try:
        return self.grid.shortest_paths[(zone1.x, zone1.y)][(zone2.x, zone2.y)]
    except KeyError:
        return float('inf')

[SeanMcOwen]

Might be better to not allow the key error since that hypothetically shouldn't be possible. I.e. if that happens then something in the simulation might be broken

[SeanMcOwen]

I.e. have the error raised or add an assertion like assert the shortest path is there and if not then stop sim