Improvements to urdf2graspit

[JenniferBuehler]

• Add a method to remove markers again in the contact points selector viewer. This can be done by re-clicking on the points for example.
• Look into improving the contacts point generation (their mathematic definition). At the moment it's fairly generic.
• Only meshes (no primitives) are supported for conversion into GraspIt format. This improvement is probably a more long-term aim.
• Mesh conversion is now done with the ivconv package, which is adapted open source code. STL files work, but inventor files as input files don't - others weren't tested. It would be a good idea to move to another library to convert the meshes.
• It would be a good idea to detach the marker generation from the actual conversion. So the robot can be converted once, and then the contact point generator can be run repeatedly on the existing result.

[JenniferBuehler]

Update: Much of the code has now moved to urdf-tools-pkgs. urdf2inventor is now the base and uses assimp to convert meshes.