Sensor parsing

[rhaschke]

This is a rebase of https://github.com/ros/robot_model/pull/125 to this new, relocated repo.

The PR enables generic parsing of sensors from URDF. The idea is to extend sensor support beyond Camera and Ray, but to allow any custom sensor description with appropriate parsers provided via plugins. We use that mechanism for example to describe tactile sensors for our tactile toolbox. For a detailed motivation, please have a look at ros/urdfdom#84.

This PR is only the very last in a whole series of PRs also involving ros/urdfdom_headers#22 and ros/urdfdom#84. Hence, it cannot compile without those.

Example usage can be seen in the unittest of tactile_toolbox.