feat(UI): Show the name of the indexes in dataframe widget

[thomass-dev]

          In a next iteration, i propose to nicely show the name of the indexes. For instance, 

Originally posted by @thomass-dev in #493 (comment)

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It should be pertinent on a dataframe with multi-indexes.

In [6]: import pandas as pd
   ...: 
   ...: df = pd.DataFrame([{"age": ">70", "number": "1M", "gender": "M"}])
   ...: df.index = pd.MultiIndex.from_arrays([["france"], ["paris"], ["1"]], names=('country', 'city', 'district'))

In [7]: df
Out[7]: 
                        age number gender
country city  district                   
france  paris 1         >70     1M      M

[thomass-dev]

@sylvaincom @MarieS-WiMLDS is it relevant for a DS to have details over multi-index?

[MarieS-WiMLDS]

yes, it can happen sometimes to use dataframes with multi-indexes due to group_by!