with_nix(): [future idea] maybe collect the log levels

[philipp-baumann]

Currently, a lot of messaging in with_nix() is controlled by helper functions and mainly by the message_type argument. Usually these are printed or not at the appropriate computation steps. Another option would be to collect all messages at a certain log level and print a selection of relevant steps to the console at the end of the with_nix() call. This might give better fine-grained control over messaging, but we will have to see if the slight extra complexity is worth it.

• Future idea: maybe there is a way to make a report so that we first collect all log levels for whatever option (verbose,...), serialise that, and serialise and maybe filter that into the current R session.

cc @b-rodrigues what we discussed at the Luxembourg meeting last week :-) just keep it as a track record issue. We can close it later if we think it is unnecessary.