Index to grouped data frame using Dicts

[pdeffebach]

Following discussion in #2265, this is the better strategy.

This PR so far has the implementation and docs, but no tests.

[bkamins]

Thank you for the fix. The design looks good to me. I have left some comments. Also function Base.to_index(gd::GroupedDataFrame, idxs::AbstractVector{T}) where {T} needs to be updated to allow Dict.

[bkamins]

As I have commented earlier this https://github.com/JuliaData/DataFrames.jl/pull/2281/files#diff-6349421a054bb7e74b7f27ae86304cbaR342 also should be fixed.

[pdeffebach]

Sorry about that. Was fixing them but got distracted.

Should be ready for another review. I added a Dict method where the keys are Strings by default. Then I added a bunch of tests.

[bkamins]

I have pushed some small things. If CI passes can you please have a final look at it and confirm all is clear and works as intended? Then I think we can approve it (but I will wait with merging for @nalimilan as he has not commented on this API change).

[nalimilan]

Sorry for the slow reaction. Looks mostly good.

[pdeffebach]

I've added AbstractDict for this, but am waiting on #2298 for the String vs Symbol output.

[bkamins]

CI still fails - can you please check the PR on Julia 1.0 and on Julia nightly to check what causes a problem?

Thank you!

[pdeffebach]

Works now!

[bkamins]

Bravo! 😄

[bkamins]

OK - so can you please just merge it with master and add NEWS.md entry please?

@nalimilan - if you do not comment on this I am going to merge this PR after @pdeffebach adds NEWS.md entry and then work on implementing the conclusions from #2305 including handling Dict.

[pdeffebach]

That was a lot easier than a rebase! I think it's ready.

[bkamins]

I have resolved merge conflict.

[bkamins]

Looks good to merge.

@nalimilan - can you please have a quick look? I would merge this after CI passes (probably tomorrow as we have a lot of CI runs queued).

[bkamins]

bump 😄 (I would finish it not to have constantly merge it with master)

[pdeffebach]

Thanks for pinging me on this! Slipped through. Thanks for the feedback Milan.

[bkamins]

also merge conflicts need resolving

[pdeffebach]

Conflict was just News.md. This can be merged.

[bkamins]

Thank you!