group-by with tree grouping feature

[genmeblog]

Disccusion: https://clojurians.zulipchat.com/#narrow/stream/264992-ml-study/topic/session.2013.2E1/near/228975717

Sketch of the solution:

• introduce new option :tree?
• It should work only in case when grouping-selector is as seq of column names: [col-1 col-2 ... col-n]
• prepare different groupings for each level, ie: [col-1], [col-1 col-2], [col-1 col-2 col-3] ... [col-1 col-2 ... col-n]
• concat result (differently for each return type).

To concat dataset, tablecloth.api.dataset/concat should be used.