def center(visible_only = true)
      if atoms.empty?
        return Atom.new(0,0,0)
      else
        bounds = bounding_box(visible_only)
        x = (bounds[0].x + bounds[1].x)/2.0
        y = (bounds[0].y + bounds[1].y)/2.0
        z = (bounds[0].z + bounds[1].z)/2.0
        return Atom.new(x,y,z)
      end
    end
def center(visible_only = true)
      if atoms.empty?
        return Atom.new(0,0,0)
      else
        bounds = bounding_box(visible_only)
        x = (bounds[0].x + bounds[1].x)/2.0
        y = (bounds[0].y + bounds[1].y)/2.0
        z = (bounds[0].z + bounds[1].z)/2.0
        return Atom.new(x,y,z)
      end
    end
Return an Atom whose coordinates are the center of the unit-cell.