When recording colisions, distinguish between agent-agent collisions and agent-environment colisions

[nickmalleson]

At the moment the collisions map produced by stationsim_gcs (e.g. model.get_collision_map()) shows most collisions happenning when agents meet the central barrier. It would be good to distinguish these collisions (which are inevitable) with those that occur when agents collide (which are the interesting ones).

[patricia-ternes]

When we calculate the collision time, we have a separation between collision with other agents and collision with station structure. So it is possible to make this distinction. However, to make the code more efficient, this information is not passed on to other methods. I will think of a way to make this distinction as efficient as possible.