csv_df = pd.DataFrame({'2016-01': [1, 2, 1], '2016-02': [0, 0, 1],
    '2016-03': [1, 1, 2], '2016-04': [0, 0, 0], '2016-05': [0, 0, 1],
    '2016-06': [0, 0, 0]})
pandas how to use groupby to group columns by date in the label?