Hover Effector

The hover effector is a pre-effector which attempts to hold an object at a set altitude. It assumes gravity (i.e. it does not have a way to make things descend), so you can think of it as an anti-gravity device. The effector takes three arguments, the target altitude, the working range (+- the target), and an exponent used to curve the velocity damping. For testing these were 1.0, 0.5, and 2.0.  There are no checks for ground impact as that is properly the job of other effectors.

Unit test is accomplished by hovering an object set at 1 meter altitude for ten seconds, then checking the min, max, and average altitudes for the last one second period. There are three runs, one starting at set point, one 1 meter above and the last 1 meter below to test the ability to capture and stabilize. In all cases the average altitude must be held to < %1 deviation to pass, and the extremes must be < 4% (< 2% when  started from set altitude). This is ready for review and merge.