Roughed out the ground effect effector (hover)

Task @T27 : Ready to start coding on hover effector.
Created the ground_effect effector "hover".

Task @T27 : Created struct hover, a ground effect device which
generates upward thrust to keep an object within a set altitude
range from ground (z=0). Takes arguments for set ride height,
min acceptable height, and an exponent (>1.0) to allow some control
of twicheyness.
Cleaned up the math in hover to remove a step.

Task @T27 : Altered hover to precompute the range between set_altitude
and min_altitude at construction, removing that unchanging step from the
tick method. This made tick less complex and perhaps a bit faster as
well.
Started work on the hover unit test.

Task @T26 : Some simple work done, mostly trying balance the
tick method so that it does not over-control. Need to add some
damping to it, I think.
Significant progress on the hover effector

Task @T26 @T27 : Have damping implemented now, looks much better.
Should complete the unit test in the next commit.
Completed hover effector unit test.

Task @T26 : Completed hover unit test. 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 devation to pass, and the extremes must be < 4% (< 2% when
started from set altitude). This is ready for review and merge.
Added hover_effector to the sim_engine Makefile.

Task @T28: Once again, I forgot to add the new module to the
Makefile for sim_engine. Corrected and tested.
Merge pull request #12 from fpstovall/ground_effect

Hover Effector is complete and ready to use.