ENH: provide keyword to groupby to reindex by all possible indicies for a multi-index

[jreback]

http://stackoverflow.com/questions/17027470/pandas-groupby-and-multiindex

not sure how useful this is....but say we provide dropna=True (which is the current behavior), but if dropna=False then can reindex by all the combinations of each index level as a reindexer...

[hayd]

This would make for a much better solution for #3820.

[jreback]

I think its the same op with 2 diff questions....but yes

[hayd]

This one however: http://stackoverflow.com/questions/16994485/ensuring-the-cartesian-product-of-keys-appears-in-a-pandas-table

[hayd]

vaguely related #3999

[jreback]

user can just use multiindex.from_product and reindex