Create panel subpackage

[kbattocchi]

This creates an econml.panel package, adds a few utilities to it, and moves DynamicDML from econml.dynamic.dml to econml.panel.dml

[fverac]

Do we want to restructure/relabel some of our docs now to include a panel section?

I believe at the moment we currently have a "Dynamic Treatment Regime" section, but maybe that can be housed under a broader "Panel Data" section? https://econml.azurewebsites.net/spec/estimation_dynamic.html

[fverac]

Do we want to restructure/relabel some of our docs now to include a panel section?

I believe at the moment we currently have a "Dynamic Treatment Regime" section, but maybe that can be housed under a broader "Panel Data" section? https://econml.azurewebsites.net/spec/estimation_dynamic.html

Bumping this. It seems to me like it could be worthwhile to rename the "Estimation Methods for Dynamic Treatment Regimes" section in our docs to something like "Estimation Methods for Panel Data" in here since this PR is sort of prepping for future panel methods.

[kbattocchi]

I've renamed the documentation section as suggested.