simulation.py: reversed order of time/state in dxdt_* methods so the controls library can use them to make system objects,
helpers.py: Added DataLog class which will track state/action variables per time step,
trajectories.py: Added loitering support so points specified as (x,y,z,T) will repeat for T steps in trajectory,
control.py: Added  attribute to all controllers to keep track of their most recent action