parallel append: suppress generation of "dta_source" variable

[johnklopfer]

Hello! Is there a way to suppress the generation of "dta_source" during a run of parallel append? This would be a useful feature when parallel is being used under memory constraints: "dta_source", because it is a file path and file name, is an arbitrarily long string. It can easily double the file size.

Thanks for all your work on the package!

[gvegayon]

Thanks for posting this! I don't think there's a way of doing that right away, what do you think, @bquistorff? PRs are welcome!