docs: def uv_at_xy(self, x, y, x0, y0, s0):
          """Returns two arrays of u, v"""
          dx, dy = self.distance(x0, y0, x, y)
          #print 'dx, dy:', dx, dy
          rr2 = (dx**2 + dy**2)**-1
          u = - s0 * dy * r_twopi * rr2
          v = s0 * dx * r_twopi * rr2
          #print 'u, v', u, v
          return u, v