[ENH] Fix summarise for MultiIndex

[samukweku]

PR Description

Please describe the changes proposed in the pull request:

• Fix error when MultiIndex is used with summarise

def dfmi():
    """Create a MultiIndex DataFrame"""

    # https://pandas.pydata.org/docs/user_guide/advanced.html#using-slicers
    def mklbl(prefix, n):
        return ["%s%s" % (prefix, i) for i in range(n)]

    miindex = pd.MultiIndex.from_product(
        [mklbl("A", 4), mklbl("B", 2), mklbl("C", 4), mklbl("D", 2)]
    )

    micolumns = pd.MultiIndex.from_tuples(
        [("a", "foo"), ("a", "bar"), ("b", "foo"), ("b", "bah")],
        names=["lvl0", "lvl1"],
    )
    dfmi = (
        pd.DataFrame(
            np.arange(len(miindex) * len(micolumns)).reshape(
                (len(miindex), len(micolumns))
            ),
            index=miindex,
            columns=micolumns,
        )
        .sort_index()
        .sort_index(axis=1)
    )
    dfmi.index.names = list("ABCD")
    return dfmi


df = dfmi()
df.summarise(('a',['min','max']),by=('a','bar'))

lvl0        a               
lvl1      bar       foo     
          min  max  min  max
(a, bar)                    
1           1    1    0    0
5           5    5    4    4
9           9    9    8    8
13         13   13   12   12
17         17   17   16   16
...       ...  ...  ...  ...
237       237  237  236  236
241       241  241  240  240
245       245  245  244  244
249       249  249  248  248
253       253  253  252  252

[64 rows x 4 columns]

This PR resolves #1459 .

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Relevant Reviewers

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• @ericmjl

[ericmjl]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.43%. Comparing base (e1b64c1) to head (0c50f47).
Report is 11 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev    #1460      +/-   ##
==========================================
+ Coverage   83.49%   92.43%   +8.94%     
==========================================
  Files          88       90       +2     
  Lines        6469     6585     +116     
==========================================
+ Hits         5401     6087     +686     
+ Misses       1068      498     -570     

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[ericmjl]

Pre-approving!

[ericmjl]

For this change, I think we may need a test. What do you think, @samukweku?

[samukweku]

@ericmjl thanks for the review ... .the default parameter is not necessary since line 504 will only kick in if line 501 is not true.

[ericmjl]

@samukweku let's merge and release!