表达持续性
    从前N1日到前N2日一直满足COND条件

    Arguments:
        COND {[type]} -- [description]
        N1 {[type]} -- [description]
        N2 {[type]} -- [description]

def LAST(COND, N1, N2):
    """表达持续性
    从前N1日到前N2日一直满足COND条件

    Arguments:
        COND {[type]} -- [description]
        N1 {[type]} -- [description]
        N2 {[type]} -- [description]
    """
    N2 = 1 if N2 == 0 else N2
    assert N2 > 0
    assert N1 > N2
    return COND.iloc[-N1:-N2].all()