* Builds the command used to provide a single tracking set of coordinates to a single servo
* @param servo_id the ID number of the single servo to send the command to
* @param trajectory_id the ID number of the trajectory the given set of coordinates belongs to
* @param point_id the ID number of the given set of coordinates inside the trajectory
* @param coordinates a vector containing the N coordinates the servo have to move to at the given time
* @param start_time only mandatory for the first point in the trajectory, a double representing the UNIX epoch of the starting instant of the trajectory