rolling window with padding.

def rolling_window(a, axis, window, center, fill_value):
    """ rolling window with padding. """
    pads = [(0, 0) for s in a.shape]
    if center:
        start = int(window / 2)  # 10 -> 5,  9 -> 4
        end = window - 1 - start
        pads[axis] = (start, end)
    else:
        pads[axis] = (window - 1, 0)
    a = np.pad(a, pads, mode='constant', constant_values=fill_value)
    return _rolling_window(a, window, axis)