Define grabber urdf

This PR updates the URDF robot description so that it matches the physical robot used in the sigproc robotic surgery project. These modifications consist of altering the shoulder part so that it has a 45 degree angle rather than a 90 degree one; and using a 4-bar mechanism for the grabber rather than the simple rotary joint in the original design. Changes which remain include adding in the final connector in the grabber, which for the purposes of the urdf is purely aesthetic. 
This PR alsomoves all the visualisation files out of the crustcrawler folder into a new smart_arm_dsc directory. The new launch file to launch from this directory is `visualise.launch`.