Adding grouping sets, rollup and cube

[clarkevans]

It would be lovely to directly support grouping sets, rollup and cube. This is related also to sorting nulls last when the sort is otherwise descending order and nulls would come first. Supporting automatic naming for aggregates is also important.

See hacked implementation at https://github.com/TuftsCTSI/TRDW.jl/commit/89cb917bbc920f390cb0b0477ae0879f2d4054d1#diff-754e6441f2054c00a80b0953b1876f78a4f3e85993eebd6fa0e3b883611a58caR182-R212