Add simplified double pendulum (#118)
* 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>