[WIP] Do Not Merge Yet. Diff drive

Add sensor and processor diff_drive.
The processor basically integrates 2D twist either from velocity commands or wheel joint displacements ( abusing the definitions : 'open-loop' / 'closed-loop' ).

There is a test_processor_diff_drive example with provided (real) data (velocities / wheel position) for which the integration seems fine.
$ ./test_processor_diff_drive data/wheel_data.dat 1
or
$ ./test_processor_diff_drive data/cmd_vel_out.dat.dat 0

It is [WIP] as it misses a diff_drive_constraint, also due to the fact that I'm not 101% sure of the jacobians in deltaPlusDelta(...) which comes from the base class ProcessorOdom2D.

Note :

the new sensor is located in a sensor directory, the new processor is located in a processor directory, opening the way to a better organization of the files.