Appending to CMAKE_MODULE_PATH

[peter-lehner]

I would like to specify my own FindQhull.cmake to your package externally, as I have the qhull include files and libraries in special paths (due to package management). This is currently not possible as you prepend your custom cmake module path to CMAKE_MODULE_PATH in https://github.com/ros-planning/geometric_shapes/blob/hydro-devel/CMakeLists.txt#L4. Would it be possible to instead append to the CMAKE_MODULE_PATH? This will only change the cmake find behaviour if someone else specifes a FindQhull.cmake file on the path as the CMAKE_MODULE_PATH is empty by default. If yes, I can provide a pull request.