[wip] Interpolator

#837 is too old to be brought up-to-date .

closes #837 , closes #266 

here's my question:
#837 would recalculate and redraw the marker every 16 milliseconds.  It used handler.postDelayed().  Where should I put handler.postDelayed() and its runnable?  It seems to me that we could put it into `GraphicDrone` and then create a DroneEvent (`newInterpolatedPosition`) which `DroneMap` listens for OR we could put the handler and runnable in `DroneMap`.