In the current setup, it is impossible to link against the ur5 or ur10 versions of the kinematics library, as all expose the same ur_kinematics::inverse() signature.
Since all libraries are exported in the catkin_package() macro, this results in undefined behavior when linking.
GCC just picks the first suitable signature it encounters, in this case the ur3 version, and continues without an error.
Adding a namespace allows the linker to find the correct library call.

Per #1 @mxgrey.

Per ROS REP 9 http://www.ros.org/reps/rep-0009.html

…amespaces