/MarkCircle2DS describes a marker of a two-dimensional circle
put in the 3D world.
/MarkCircle2DS corresponds to class G4Circle of GEANT4 with
its size given as screen size.
| Format | /MarkCircle2DS x y z r |
| (x, y, z) | marked 3D position |
| r | marker size (2D unit): radius of the circle |