* alex description added

* modified meshes - size reduced - readme and license added

* Add go2 description

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

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- always try to import EXAMPLE_ROBOT_DATA_MODEL_DIR and EXAMPLE_ROBOT_DATA_SOURCE_DIR

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Removed since EigenPy 3

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Adds BSD-3 licensed ANYmal C simple description from
https://github.com/ANYbotics/anymal_c_simple_description commit
f119c8f3c4b9e08948b0f5915e9c7cedf63f52e5

Modified to:
1. Fix the path for mesh loading
2. Add SRDF

* cassie/cassie_v2 sdf: remove first primatic joint, add color information

* robots_loader: use updated sdf parser for pin version > 2.9.1

* cassie srdf: update joint info, add missing joints

Co-authored-by: Rohan Budhiraja <proyan@users.noreply.github.com>

* Add simplified double pendulum

* Add description to urdf, loader and unittest

* Fix format

* Fix format for unittest

* double pendudum: update credits

Co-authored-by: Justin Beri <justin [dot] beri [at] csiro [dot] au>
Co-authored-by: Justin Beri <justin.beri at hdr.qut.edu.au>
Co-authored-by: Guilhem Saurel <guilhem.saurel@laas.fr>