Results of HRI code sprint for stereo-based road area detection (stereo module)

Here is the first part HRCS' results (only for the stereo module).

Add in module stereo:
* DisparityMapConverter class for converting disparity maps to point clouds.
* DigitalElevationMapBuilder class for building Digital Elevation Maps from disparity maps.

Add in module common:
* A PointDEM point type for representing Digital Elevation Maps.
Shouldn't thresholds be set in the constructor? Changing them invalidates the data accumulated so far (because `step_` changes).