message: particle volumes work in progress
diff: namespace ospray {
  namespace sg {
  -struct ParticleImporter : public Importer
  +struct ParticleVolumeImporter : public Importer
  {
  - ParticleImporter() = default;
  - ~ParticleImporter() override = default;
  + ParticleVolumeImporter() = default;
  + ~ParticleVolumeImporter() override = default;
  void importScene() override;
  };
  -OSP_REGISTER_SG_NODE_NAME(ParticleImporter, importer_particle);
  +OSP_REGISTER_SG_NODE_NAME(ParticleVolumeImporter, importer_particle);
  // ParticleImporter definitions /////////////////////////////////////////////
  -void ParticleImporter::importScene()
  +void ParticleVolumeImporter::importScene()
  {
  // Create a root Transform/Instance off the Importer, then place the volume
  // under this.
  