/MarkCircle2D describes a marker of a two-dimensional circle
put in the 3D world.
/MarkCircle2D corresponds to class G4Circle of GEANT4 with
its size given as world-coordinate size.
| Format | /MarkCircle2D x y z r |
| (x, y, z) | marked 3D position |
| r | marker size (3D unit): radius of the circle |