Conversion method to Pandas DataFrame. To be attached to ResultDict.

    Returns
    ========
    List : of Pandas DataFrames in order of Total, First, Second

def to_df(self):
    '''Conversion method to Pandas DataFrame. To be attached to ResultDict.

    Returns
    ========
    List : of Pandas DataFrames in order of Total, First, Second
    '''
    total, first, (idx, second) = Si_to_pandas_dict(self)
    names = self.problem['names']
    ret = [pd.DataFrame(total, index=names),
           pd.DataFrame(first, index=names)]

    if second:
        ret += [pd.DataFrame(second, index=idx)]

    return ret