Adding a new model

[Virgile-Foussereau]

Hello,

We have been trying to add a bicycle model to the package, without success. Our current progress is visible on this fork. Basically, we started from the unicycle model and we just added a new variable called steering, which is used in the predictions. We made sure to create all appropriate model files corresponding to the new state of size 9 (was 8 before adding steering).
However, when running it the state and the jacobians quickly become extremely large values. Our guess is that some other part of the repo is not adapted for this new model of state size 9 but we have a hard time finding what is going on.

Do you have any idea or suggestion? We would be happy to contribute the bicycle model to the repo afterward if we manage to make it work.