What if we just get rid of the special treatment for flat terrain? This seems like something that would make sense:
* `options.terrain` unspecified => no terrain
* `options.terrain = RigidBodyFlatTerrain` => flat terrain
* `options.terrain = RigidBodyHeightMapTerrain` => height map terrain
* etc.
That way users who want to construct manipulators with non-flat terrain can do so without having to do
```
r = RigidBodyManipulator(...)
r = setTerrain(...)
r = compile(r);
```
Then all the `options.terrain = []` would go away. Instead, the files that need flat terrain would need to have `options.terrain = RigidBodyFlatTerrain` in them, but at least that's a meaningful statement.