NMPC Swing Up Controller based on Crocoddyl

[pierfabre]

• Rebased on generate an urdf of the robot #93+
• Add a NMPC controller for swing up that uses crocoddyl library and mim_solver
• scripts/nmpc_swing_up.py in which we solve the TO offline first and then try to swing up the pendulum in a MPC fashion using the new simulator better simulation #95+. This is already challenging because a lot of things are not in the model (friction, encoders resolution, control signal clamp, velocity filters, smaller integration step, ...)

Realated to #66

[pierfabre]

@Armandpl the test_gym_env test is failing because the reward is of type numpy.float32 and the env_checker expects float
It has nothing to do with the PR
I think it's probably due to the bump in version of numpy thanks to the relaxation on matplotlib version
Can you take a look pls?