Estimate special groupby modes

not sure if its handled already..but it looks like there is some underestimation; if the groups are not counted in - that could lead to serious underestimation if there are a lot of groups

ql/src/test/results/clientpositive/llap/vector_groupby_grouping_sets2.q.out
 SELECT a, b, sum(c+d) from T2_n42 group by a, b with cube