Add AFM.move_toward_surface() and fix AFM.move_away_from_surface().

Add AFM.move_toward_surface() and fix AFM.move_away_from_surface().

In everything else we do, numbers increase as you approach the
surface.  There's no need to invert that convention here, so we now
have `AFM.move_toward_surface()` which can be used for moving in both
directions.  `AFM.move_away_from_surface()` remains, because "Ahh!
What's going on!  Quick, save the tip!" moments will happen, and then
it's nice to have a method that gets you clear without you having to
pick how far away you'd like to go.