Add cassie robot in example-robot-data (#100)

* [cassie_description] initial commit

* [cmake] update

* [cassie.sdf] change mesh path relative to example-robot-data

* [cassie_description] move meshes up folder

* [cassie-loader] create loadCassie function for load with sdf

* cassie srdf: standing pose

* [cassie_loader] use robotloader

* add unittest. won't yet work before pinocchio3

* Update unittest/test_load.py

Co-authored-by: Guilhem Saurel <guilhem.saurel@laas.fr>

* ci fixes: flake8 + unittest missing module

* fix test

Co-authored-by: Rohan Budhiraja <proyan@users.noreply.github.com>
Co-authored-by: Guilhem Saurel <guilhem.saurel@laas.fr>