Allow AsTable on RHS in transformations

[pdeffebach]

After the recent improvements to AsTable in DataFrames, I guess it's time to make it easier to work with this construct in DataFramesMeta.jl.

No tests added yet.

[bkamins]

What would be the test cases for this?

[pdeffebach]

a row-wise sum

cols = [:a, :b, :c, :d]
@rtransform df :y = sum(AsTable(cols))

[bkamins]

Nice - probably documentation should be updated (or does it already have it?)

[pdeffebach]

No I haven't done any of that yet. I'll ping you when its ready.

[pdeffebach]

Okay I've added enough stuff so that you can see the API

julia> df = DataFrame(rand(10, 10), :auto);

julia> @rselect df :sum_all = sum(AsTable(names(df)))
10×1 DataFrame
 Row │ sum_all 
     │ Float64 
─────┼─────────
   1 │ 4.19633
   2 │ 4.69652
   3 │ 2.96451
   4 │ 4.32973
   5 │ 4.69788
   6 │ 3.68512
   7 │ 6.47631
   8 │ 5.80387
   9 │ 4.92043
  10 │ 4.83637

[pdeffebach]

Okay ready for a review @bkamins @nalimilan

If only to check if the API makes sense.

[nalimilan]

Just a few stylistic comments.

[pdeffebach]

Thanks for the review!

I have addressed all comments. I also got rid of the code example in the table because I couldn't figure out a way to get it to work.

I think this is ready to merge?

[bkamins]

Looks good. I made some cosmetic comments.

[nalimilan]

Again, why not put this section right after the one about @astable?

[pdeffebach]

Thanks. Re-arranged.

[pdeffebach]

Thanks for the review. I addressed all comments.

[pdeffebach]

Thanks. Re-arranged.

[pdeffebach]

Okay ready for review / merging.

[bkamins]

I went through the comments and resolved all that I thought were addressed. I left one related to indentation (a minor issue) + one regarding the order of presentation of things (so that @nalimilan can check if now we have what he wants).

Thank you for working on thins!

[pdeffebach]

Awesome! Thanks for all the hard work reviewing this.