Description

allow log_fmt_shell to log to two files.
This is for the use case if the log file of a script needs to be used without any sterr from the wrapper.

  If you provide two log files, name them err/out or sterr/stout.
  The function returns:

      2>> sterr > stout
      or with append=True
      2>> sterr >> stoud 

Appending to sterr is required as error messages from the wrapper will be overwritten otherwise.

Its used in snakemake/snakemake-wrappers#1322

QC

• The PR contains a test case for the changes or the changes are already covered by an existing test case.
• The documentation (docs/) is updated to reflect the changes or this is not necessary (e.g. if the change does neither modify the language nor the behavior or functionalities of Snakemake).