Sign error in get_id_from_name in robot-utils

[gabrielebndn]

Classes RobotUtil and ForceUtil have a get_id_from_name method which returns an id number given a name string. If the name is not found, -1 is returned.
However, the return type is Eigen::VectorXd::Index, which is a typedef for unsigned int.
Code using this methods checks that the result is less than zero, which will never work.

This is obviously a mistake. The return type should be int.

By the way, I do not really understand the need of these methods. We could directly work with Pinocchio models and use its methods.

[olivier-stasse]

They are historical and comes from a time where pinocchio was not around.
They are currently being revaluated.