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 to set, accepts either a numeric or a callable object (a function) which returns a numeric.
The trajectory object.
Other methods for dealing with trajectories: create_trajectory, get_head, get_tail, get_n_activities, join, seize, release, timeout, branch, rollback, leave, seize_selected, release_selected, select.