docs: def step_forward_with_function(self, uv0fun, uv1fun, dt):
          """Advance particles using a function to determine u and v.
          
          Parameters
          ----------
          uv0fun : function
              Called like ``uv0fun(x,y)``. Should return the velocity field
              u, v at time t.
          uv1fun(x,y) : function
              Called like ``uv1fun(x,y)``. Should return the velocity field
              u, v at time t + dt.
          dt : number
              Timestep."""
              
          dx, dy = self._rk4_integrate(self.x, self.y, uv0fun, uv1fun, dt)
          self.x = self._wrap_x(self.x + dx)
          self.y = self._wrap_y(self.y + dy)