Fix terrain maps

The USE_MAPS compiler flag was never getting set, and the maps code was never used (and was also incompatible with the API changes from this summer). I've fixed that to set the flag if needed and updated the code. We can now walk on DRC terrainmaps in Drake. 

You can test this out in the DRC drakeWalking.m script by setting `xstar(3)` to some non-zero value and then adding the following lines after the call to `r.constructVisualizer()`:

```
r = r.setTerrain(DRCTerrainMap(false,struct('name','Foot Plan','status_code',6,'listen_for_foot_pose',false)));
r = r.compile();
r = configureDRCTerrain(r, drc.footstep_plan_params_t.HORIZONTAL_PLANE, xstar(1:r.getNumPositions));
```