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

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 apps:
* DEMGroundSegmentation class for road area segmentation, using DEMs.
* apps/src/dem_ground_segmentation_example.cpp - file with example of usage.

The code depends from 3d-party libraries NLopt and OpenGM. That libraries have BSD-compatible licenses. Also, cmake modules FindNLopt.cmake and FindOpenGM.cmake were added.