Split-apply-combine todo

[bkamins]

An issue to keep track of the things that are pending todo in split-apply-combine ecosystem:

• decide on behavior of map (but let us discuss it in a separate thread)
• adding handling dropmissing kwarg
• handling of check for preserved index column during combine #1460 (we still do not handle it)
• handling of mixing scalars and vectors
• writing test-cases for hard scenarios - especially for mixing different things in return values and duplicate column names
• extending by interface by with arrays of inputs and broadcasting #1615
• check performance of categorical vs string for grouping https://discourse.julialang.org/t/dataframes-v0-15-0-released/18258/10
• consider adding a special code path for the case when a data frame is already sorted (added)

@nalimilan - please add if I missed anything

[bkamins]

The last point (categorical vs string as a grouping variable) also influences how we should set up tests in h2oai/db-benchmark#51.