[Matrix]: implement groupby cols on a tall matrix.
[Matrix]: implement groupby cols on a tall matrix.

icoming/FlashGraph,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,flashxio/FlashX,flashxio/FlashX,icoming/FlashX