Less falling AI & fixed/adjusted detect functions

Enemies check if their next waypoint is over ground before moving.  This
makes them less likely to run off of edges.  They can still get close
enough that they run off while turning.

Adjusted the detection function to use a Vector 3 rather than an object,
and made it take the full width of a side of the area, rather than half
the length of a side.  These changes allow for a more general use and
simpler math.  I also added a second, similar version that takes a
Vector 3 specifying the X, Y, and Z widths of the area separately.  This
allows for rectangular areas.

I also fixed the enemy detection, so enemies should be moved and
destroyed by creating cubes again.