Wheels follow ground with heightmaps

Also adds a `setHeading` method that can be used to simultaneously set the rover's heading and rotation, rather than making the scenario creator do both and keep them in sync.

@BrettASwift @kadst43  would you mind reviewing?
It does!  This is new.  @davideaster recently added support to the core for dealing with nodes as properties.  So, this is the first time something like this is possible.  I think we should start using it all over the place.  :smile: 

High, five, @davideaster :smile: