Add support for hierarchical multi-indexed DataFrame

[philippjfr]

This PR improves the DataFrame widget in a two ways relating to support for DataFrame indexes:

1. It fixes a bug editing DataFrames with a non-default index name
2. It adds support for displaying hierarchical multi-indexed dataframes with aggregators

As an example here is a three-level DataFrame aggregated over the indexes:

from bokeh.sampledata.population import data as population_data 

pop_df = population_data[population_data.Year == 2020].set_index(['Location', 'Sex', 'AgeGrp'])[['Value']]

pn.widgets.DataFrame(value=pop_df, hierarchical=True, aggregators={'Sex': 'sum', 'Location': 'sum'}, height=400) 

[codecov]

Codecov Report

Merging #1383 into master will increase coverage by 0.07%.
The diff coverage is 92.75%.

@@            Coverage Diff             @@
##           master    #1383      +/-   ##
==========================================
+ Coverage   87.68%   87.75%   +0.07%     
==========================================
  Files         130      130              
  Lines       13928    14002      +74     
==========================================
+ Hits        12213    12288      +75     
+ Misses       1715     1714       -1     

Impacted Files	Coverage Δ
panel/widgets/tables.py	85.40% <92.53%> (+5.25%)	⬆️
panel/tests/widgets/test_tables.py	97.19% <100.00%> (+0.64%)	⬆️
panel/widgets/base.py	89.88% <0.00%> (+2.24%)	⬆️

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[jbednar]

Cool!