Initial state of the bridge class we discussed

 - It required to adapt the steer API we talked few weeks ago.
 - The changes in StatePropagator.h are provisional, to avoid compilation errors due to the API modification.
 - Please, take a look at the TODO, there are many of them which are important so I do not want to continue until discussing them.

Main problems:
 - the distance function. Steer returns total duration of the control but most of the time this is not the metric to use.
 - When interpolating, the final state will always have an error, I cannot find a way of avoiding the use of a step size variable. I kept it this way for simplicity but tried some other things. This is important for optimizing planners.