proc_path_gen: proposed fixes for build issues

As per title.

This uses a pkg-local version of a CMake find script for `openvoronoi`. I'm not sure whether this should be exported using the appropriate `catkin_package()` argument. The find script is basically a copy of `FindTinyXML.cmake` in `cmake_modules`.

One open issue is the dependency on (Boost) Python: without the `python` component, linking of `process_visualization_node` fails. Not sure where that is coming from, it happens even when `openvoronoi` is installed without Python bindings.