add basic_detector.gdml based on original raindrop geometry

[plexoos]

The key definition for the optical surface properties of the container walls is provided in the following line:

<opticalsurface finish="polished" model="glisur" name="medium_container_bs0x560299397a10" type="dielectric_metal" value="1">

https://github.com/Geant4/geant4/blob/32390e802bd84a8b85ac02a90a3d31cf205dd5b7/source/materials/include/G4SurfaceProperty.hh#L58-L68

enum G4SurfaceType
{
  dielectric_metal,       // dielectric-metal interface
  dielectric_dielectric,  // dielectric-dielectric interface
  dielectric_LUT,         // dielectric-Look-Up-Table interface
  dielectric_LUTDAVIS,    // dielectric-Look-Up-Table DAVIS interface
  dielectric_dichroic,    // dichroic filter interface
  firsov,                 // for Firsov Process
  x_ray,                  // for x-ray mirror process
  coated                 // coated_dielectric-dielectric interface
};

https://github.com/Geant4/geant4/blob/32390e802bd84a8b85ac02a90a3d31cf205dd5b7/source/materials/include/G4OpticalSurface.hh#L52-L60

enum G4OpticalSurfaceFinish
{
  polished,              // smooth perfectly polished surface
  polishedfrontpainted,  // smooth top-layer (front) paint
  polishedbackpainted,   // same is 'polished' but with a back-paint


  ground,              // rough surface
  groundfrontpainted,  // rough top-layer (front) paint
  groundbackpainted,   // same as 'ground' but with a back-paint

https://github.com/Geant4/geant4/blob/32390e802bd84a8b85ac02a90a3d31cf205dd5b7/source/materials/include/G4OpticalSurface.hh#L106-L113

enum G4OpticalSurfaceModel
{
  glisur,   // original GEANT3 model
  unified,  // UNIFIED model
  LUT,      // Look-Up-Table model (LBNL model)
  DAVIS,    // DAVIS model
  dichroic  // dichroic filter
};

[plexoos]

Consider various roughness: 0.9, 0.6, 0.3, and 0

[plexoos]

dielectric_dielectric

dielectric_metal (reflectivity 0.5 -> 0.99)

dielectric_metal (reflectivity 0.99, roughness=0.6)

[plexoos]

Next tests:

• add reflective surfaces of various shapes
• add a (small) detector volume to register hits with both G4 and GPU simulations

cc @ggalgoczi