singlediode.bishop88_mpp return is different from docstring

[cwhanse]

Docstring states

    Returns
    -------
    OrderedDict or pandas.DataFrame
        max power current ``i_mp`` [A], max power voltage ``v_mp`` [V], and
        max power ``p_mp`` [W]

This function returns a tuple (i_mp, v_mp, p_mp)