Movement and terrain

Added a "selector" indicator to show where you will be modifying
terrain, messed around with movement. There's acceleration/deceleration
now, and you actually collide with the terrain. If the height difference
is small enough, it will propel you upwards. This way you can walk up
slopes. There's also a "get averaged height" function now that attempts
to interpolate height values between voxels. The results are acceptable
for now.