Groupby all levels except the specified ones
Inspired by 

> http://stackoverflow.com/questions/25606478/grouping-by-everything-except-for-one-index-column-in-pandas

 I was thinking if it would be useful to add a parameter to the groupby function (e.g. allbut=True) so that pandas will group all levels except the specified ones) .

I suggest so because in a multilevel index with several levels, eliminating just one level requires a quite verbose approach 