def add_plane_cartesian(nx, ny, nz, x, y, z)
      add_plane(Plane.new(nx, ny, nz, x, y, z), true)
    end
def add_plane_cartesian(nx, ny, nz, x, y, z)
      add_plane(Plane.new(nx, ny, nz, x, y, z), true)
    end
Add a clipping plane defined by the outward normal (nx,ny,nz)
 and a point on the plane (x,y,z)
 Only points behind the plane will be kept.  Points in front
 of the plane will be invisible.  They are not really gone, but
 can be returned by moving or removing the plane