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[c++] Add asserts in RobotWrapper methods to check whether joint index and frame index are within bounds. Replace deprecated method pinocchio::framesForwardKinematics with its new version.

[python] Fix a few bugs introduced with recent changes to make ex_3 work with Talos and meshcat viewer.

merge devel to capture-point for pull request

to fix build of summer-school website:
> The submodule registered for `./cmake` could not be cloned.
> Make sure it's using https:// and that it's a public repo.
> For more information, see https://help.github.com/en/github/working-with-github-pages/troubleshooting-jekyll-build-errors-for-github-pages-sites#invalid-submodule.

[python] Update notebooks and scripts to use urdf files from example-robot-data. Update notebooks to use numpy.array instead of numpy.matrix. Cleaning up.

Add option in tsid_biped.py to specify contact motion constraints as costs rather than hard constraints.

[bindings] Add python binding for method to add 6d contact with motion constraint at priority level 1 (i.e. as a cost)

thanks @paleziart

* Function setGravity added
Co-authored-by: Aurélie Bonnefoy <abonnefoy@laas.fr>