DifOdo integration with MRPT + 2 examples OpenNI2

DifOdo (differential odometry) class is added (CDifodo)
2 applications are included to use it:
- "Difodometry-datasets" to test its performance with the datasets from
Freiburg/TUM.
- "Difodometry-camera" to perform visual odometry in real time with an
RGB-D camera.

Independently, 2 examples are added to illustrate how to manage a RGB-D
camera with the OpenNI2 middleware.