Added a ZO 4wd Differential Drive Controller

[ghost]

For the needs of the LeoRover. A differential drive controller that controls 4 wheels was required. We modified the ZODifferentialController for this purpose. There might be a better way to achieve this, but it works.

We added a SimpleAMR4wd and a test scene to prove that the script is functionnal.

I also added some instructions in the ReadMe on how to modify the source of this package.