[Matrix]: implement get_cols and set_cols in mem_sub_col_dense_matrix.
[Matrix]: implement get_cols and set_cols in mem_sub_col_dense_matrix.

icoming/FlashGraph,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashX,icoming/FlashGraph,icoming/FlashGraph,zheng-da/FlashX,icoming/FlashX,icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,icoming/FlashX,icoming/FlashGraph,zheng-da/FlashX,flashxio/FlashX,zheng-da/FlashX,flashxio/FlashX