def miller_from_cartesian(x, y, z)
      if self.cart_to_miller
        self.cart_to_miller*Vector[x, y, z]
      else
        nil
      end
    end
def miller_from_cartesian(x, y, z)
      if self.cart_to_miller
        self.cart_to_miller*Vector[x, y, z]
      else
        nil
      end
    end
Given a vector (x,y,z) in cartesian coordinates,
 return the miller-index corresponding to that vector's direction