Clustergram label is obscure and lengthy.

[mkcor]

Hi @shammamah,

As I was linting app_clustergram.py, I noticed some heavy and confusing wording, lines 162-164:

'Header of row labels column in uploaded dataset',
title='If a dataset was uploaded, enter the header of' +
'the column that contains the title of each row.',

Even when I view it in context, it's unclear what this label means (I mean 'label' in the context of an app layout).

In Pandas language, when talking about a DataFrame, row labels are referred to as the index and column labels as columns.
Reference: https://pandas.pydata.org/pandas-docs/stable/dsintro.html

Maybe we could follow this convention, instead of torturing our brains?