Add collision number of trajectory to output

[Nhertl]

The number of collisions between an adsorbate with the closest neighbouring substrate during a trajectory would be adjuvant for studying the scattering dynamics. Introducing this keyword would allow categorising trajectories into single bounce, double bounce or multibounce trajectories which in turn facilitates further analysis of those categories regarding, for instance, final kinetic energy distribution, or quantum states.

The appropriate criterion of a collision has, however, a slight taste of arbitrariness to it because different criteria lead to different results.

The most meaningful criterion, IMHO, would be a hard-sphere criterion but the downside is that prior knowledge of the system of interest is required, in order to pick a meaningful hard-sphere radius.