Warn users if they have cohortextractor actions with uncompressed csv outputs

[bloodearnest]

Probably with link to documenation.

[StevenMaude]

This could well be a useful 🩹 to help dissuade people from using plain CSV.

I haven't run enough projects to know how visible would such a warning be, in the midst of other log output? Is it likely to be seen?

Another and/or additional possibility is having the OpenSAFELY CLI more centrally audit a project, and check for antipatterns. At the moment, it's possible to have CSV output from Data Builder, which might cause similar problems in future.

[bloodearnest]

Good point, doing it in opensafely-cli is probably better. Moving it there.