message: call ospShutdown on python exit and plug a leak
  The call to ospShutdown happens automatically, users do not need to care.
  The call to sg.clearAssets() is done in batch and gui mode already, it just
  frees a map and thus plugs a leak.
diff: @@ -212,11 +212,18 @@ std::shared_ptr<Data> pysg_Data(const py::array &array, bool is_shared = false)
  // Main SG python Module ///////////////////////////////////////////////////
  +auto cleanup_callback = []() {
  + sg::clearAssets();
  + ospShutdown();
  +};
  +
  PYBIND11_MODULE(pysg, sg)
  {
  // OSPRay initialization
  sg.def("init", &init);
  + sg.add_object("_cleanup", py::capsule(cleanup_callback));
  +
  // Main Node factory function ////////////////////////////////////////////
  