API: Panel.groupby has an older interface
This seems to not conform to the `DataFrame.groupby/Series.groupby`

should be straightforward to fix this I think, just simply removing it in favor of `core/generic/NDFrame.groupby`, but would need to fix tests, maybe. The `mapper` argument is the `by` argument.

http://stackoverflow.com/questions/24914748/why-does-panel-groupby-not-accept-level-parameter-in-pandas-0-14/24915441#24915441
