No longer internally call .reset_index()

[maximlt]

HoloViews is going to better support Pandas indexes in holoviz/holoviews#6061 so it doesn't have to internally call .reset_index() when a reference is made to an index column. This change will come with some great performance improvements. hvPlot itself calls in several places .reset_index(). To fully benefit from HoloViews' improvements, hvPlot will have to remove these calls to .reset_index().

[maximlt]

Bugs identified and fixed when working on addressing this:

• Ensure date sorting of multi index Pandas dataframes #1363
• Ensure no auto index is created when a kdim is part of a multi index holoviews#6299

[droumis]

Closed by #1350