add a `groupby` method

[rafaqz]

Add a groupby method to DataAPI so it can be shared accross packages like DataFrames.jl and DimensionalData.jl.

See: https://github.com/rafaqz/DimensionalData.jl/pull/591

[bkamins]

Also a bump in package version is needed for the release.

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[nalimilan]

Does it really make sense to have a docstring that says so little? Also, docstrings are exposed to users, so indications intended only at implementers would better go elsewhere. For join functions (leftjoin , etc.), we currently don't provide any docstring, and instead use a comment inside the code.

[bkamins]

I am OK to turn this into a comment.

[rafaqz]

Any objections to adding aggregate here at the same time for the same reason?

[bkamins]

What packages use aggregate currently? DataFrames.jl does not use this name.

[rafaqz]

Really?

julia> aggregate
WARNING: both Rasters and DataFrames export "aggregate"; uses of it
 in module Main must be qualified

~Maybe a reexport?~ oh its deprecated.

But in practice that amounts to the same thing for users of both packages.

[bkamins]

We will remove aggregate in the next release of DataFrames.jl then.

[bkamins]

OK. @nalimilan - the state is that we export by and aggregate. I think no one uses them, we deprecated them many years ago.

Would you be OK to remove:

export by, aggregate

# TODO: remove definitions in 2.0 release
by(args...; kwargs...) = throw(ArgumentError("by function was removed from DataFrames.jl. " *
                                             "Use the `combine(groupby(...), ...)` or `combine(f, groupby(...))` instead."))
aggregate(args...; kwargs...) = throw(ArgumentError("aggregate function was removed from DataFrames.jl. " *
                                                    "Use the `combine` function instead."))

before 2.0 release (it is technically breaking, but in practice I think it is not problematic to drop it.

[nalimilan]

Sure.

[bkamins]

Released