feat: ak.from_rdataframe should accept a single string 'columns'.

[jpivarski]

This adds a guard, but it also introduces a way of calling ak.from_rdataframe (which was already partially in the documentation anyway).

It's to be expected that users will sometimes pass a single string into columns. When this happens, we could

• raise an error to let them know that's wrong (it's already doing that, but the error message could be more clear)
• turn a single string x into (x,), squashing the distinction between a string and a length-1 tuple of strings
• use the single string interface to mean "I want an Awkward Array of a column" (what this function did before feat: retrieve multiple columns from RDataFrame in a single event loop #1625) and the collection of strings (even if there's only one of them) to mean "I want an Awkward Array of multiple columns." The latter will always be a record array, and if the collection has only one string in it, the record array will have only one field in it.

In this PR, I opted for the third case. It seems to me to be the most useful one.

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📚 The documentation for this PR will be available at https://awkward-array.readthedocs.io/en/jpivarski-from_rdataframe-should-accept-a-single-string/ once Read the Docs has finished building 🔨

[codecov]

Codecov Report

Merging #1956 (a2437fa) into main (d1ff4f1) will decrease coverage by 0.04%.
The diff coverage is 0.00%.

Additional details and impacted files

Impacted Files	Coverage Δ
src/awkward/operations/ak_from_rdataframe.py	17.64% <0.00%> (-25.22%)	⬇️
src/awkward/operations/ak_where.py	91.66% <0.00%> (-0.18%)	⬇️
src/awkward/_util.py	81.00% <0.00%> (-0.15%)	⬇️
src/awkward/operations/ak_broadcast_arrays.py	100.00% <0.00%> (ø)

[ianna]

Looks great! Thanks.

[agoose77]

That's all reviewers! I'll merge.