* 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>

* Make double_pendulum's joints unbounded revolute (continuous)

* Add continuous (SO(2)) variant of double_pendulum

* Remove update to deprecated function; inherit from DoublePendulumLoader

* Add unit test

* robots: document upstreams & license, fix #34