Simple mechanism to redirect logfile to std::cerr

[ykempf]

Every now and then when cerr-debugging I end up either crossing my eyes between logfile.txt and the slurm output file, or manually moving logfile output to cerr so I have a consistent chronological output sequence in one file. Using logfile for debugging is not practical e.g. when adding cerr lines in libraries.

It would be convenient if a cfg or compilation/preprocessor flag allowed to easily switch over all logfile output to cerr.

Posting here as I'm definitely not implementing this today, but if someone has a solution and posts it here, I or they can push it next time it gets used.