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.
This does not seem to be a trivial class, so I would suggest to move it to a separate file. Furthermore, if this is of a general usage, then I would even put it into a different module (`common`?). Also, if this a part of the public interface of your classes? Or an implementation detail?