Edge order in np.gradient

[jruebsam]

For the taylor expansion of the velocity fields currently no edge order is used i.e.:

    def _get_displacement_function(self, f):
        dx = self._distance
        f_x,  f_y  = np.gradient(f  , dx)
        f_xx, f_xy = np.gradient(f_x, dx)
        f_yx, f_yy = np.gradient(f_y, dx)
        return lambda i, j, x, y : f[i, j] + x*f_x[i, j]  + y*f_y[i, j]#\

maybe it is better to add the additional edge_order argument to np.gradient?