Robot class should have API for getting joint limits

[ZdenekM]

Right now, ARServer is not able to check, if manually set joint values are within an acceptable range. The Robot class should have API for getting those. Those could be by default read-out from a robot's URDF for example. There should be a possibility to override that method (hardcoded values, getting it through REST call).

There are some URDF-related libraries around:

• https://github.com/mmatl/urdfpy
• https://github.com/ros/urdf_parser_py
• https://github.com/Phylliade/ikpy