The main contribution of this PR is to implement in c++ the sympletic Euler integrator with its Python bindings. Additionally I have included plenty of extra tasks:
- Removed inconvinient wrapper classes (for actions and state vector). Otherwise we cannot retrieve the pointers of such classes (such as in the method differential of Euler integrator). For this, I have to add Eigen::VectorXd/MatrixXd-based functions inside abstract classes.
- Fixed a bug in the differential LQR.
- Added the unit-test of the differential LQR.
- Moved the numdiff files into a specific folder. @mnaveau I have decided a tiny modification in the organization. I will handle these stuffs.