Fix batch minute

    Before the change to the RollingPanel, window_length
    specified the number of days that should be in a window.
    
    The previous commit broke this if data was minute resolution.
    
    By passing bar='minute' to the batch_transform we internally
    multiply the window_length by 60*6.5 to have a full day.
    
    Also adds a (still rudamentary) test for batch_transform
    with minute data.