Adds a new key/value attribute. The value should be numeric.
set_attribute(traj, key, value)the trajectory object.
the attribute key (is coerced to a string).
the value (should be numeric or a function which returns a numeric).
The trajectory object.
Other methods to deal with trajectories: create_trajectory, get_head, get_tail, get_n_activities, seize, release, timeout, branch, rollback.