Remove array index offset

[ecbrown]

Fix proposed in #41

[vmbecerra]

Dear Eric

Thank you so much for your suggestion.

I have realised that the *_propagate functions are difficult to use and fairly restrictive. Moreover, as you noted they are not working well. There is actually a better way of propagating a user-supplied control input by solving an optimal control problem with a special objective functional. This allows to include path constraints, which were not possible to consider using the 'propagate' functions. I will document this alternative approach instead and remove the propagate functions, so I will not approve this particular merge request as it will not be needed anymore.

Best wishes

Victor.