Align Python Jacobian to C++, fix #545

bindings/python: correct bindings of contact dynamics

cmake: add check of submodule loading

[Python] Improve conversions and usage of gepetto-gui API.

Introduce support for MeshCat viewer in RobotWrapper.

 - RobotWrapper can now be used with both gepetto-gui and MeshCat as 3D viewer.
 - Add example script meshcat-viewer.py for show how to display a robot in MeshCat.
 - Fix gepetto-viewer.py example script that was using a deprecated API.

Define contact-dynamics and minor cleanings

joints: fix return type of translation() or rotation() methods