Use MultiIndex for ExtraP Functions

[michaelmckinsey1]

The componentize_statsframe and _add_extrap_statistics extrap model functions both add a lot of columns to the statsframe.

Statsframe

avg#inclusive#sum#time.duration_extrap-model	avg#inclusive#sum#time.duration_RSS_extrap-model	avg#inclusive#sum#time.duration_rRSS_extrap-model	...
cell	cell	cell	cell
cell	cell	cell	cell

Currently, we are appending strings to the column name to distinguish them between each other. This becomes very messy fast and is not user friendly. I propose pulling out the common strings between the columns, like avg#inclusive#sum#time.duration, into a higher-level column index, and let the different strings distinguish each column, like model, RSS, rRSS.

And what if our statsframe already had other statistics and/or models in it? Our problem would only become worse. So I believe we can leverage multi-indexing to help us organize the statsframe.