Utheque: URDF-theque (library of URDFs)

[jbmouret]

Objective: make it easy to use the well-maintained URDF files without robot_dart (e.g., on a real robot, which does not have assimp, dart, etc.).

• We install the URDF files in $PREFIX/share/utheque instead of $PREFIX/share/robot_dart so that it makes more sense to use the same URDF if we do not care about robot_dart (e.g., for controllers)
• We can install the URDF even if robot_dart does not compile
• There is a small header-only library utheque/utheque.hpp that makes it easy to get a URDF from the library (e.g., in a controller)

TODO: option to deactivate explicitly robot_dart

[costashatz]

Overall this is nice! See a few minor comments. Once they are done, I will format the code and merge! Thanks!

[jbmouret]

Please note that I did not try the CMake integration yet.

[costashatz]

@jbmouret CMake integration should be working now. I tried on my PC and added a test in the CI. Check also in your Mac. If it works there as well, we should be ready to merge...