/MarkSquare2D describes a marker of a two-dimensional right square
put in the 3D world.
/MarkSquare2D corresponds to class G4Square of GEANT4 with
its size given as world-coordinate size.
| Format | /MarkSquare2D x y z dx |
| (x, y, z) | marked 3D position |
| dx | marker size (3D unit): half edge length of the square |