Csv helper

After having written this, I'm not convinced it's a real improvement. This doesn't avoid the problem of needing to load the entire file into RAM at once; it doesn't fix the bug that comes when you have two headers with the same name (though now I see how to fix that). And my dropped column was because the file was UTF-16, so this doesn't improve matters.

I'm submitting the MR in case you're curious, but until I resolve one or more of those issues, I doubt it's worth merging and you can close this whenever you feel like.