Allow specific log filenames to be supplied to ekat::logging::Logger

[jeff-cohere]

Is your feature request related to a problem? Please describe.

Currently, all of the constructors for EKAT's Logger class append the MPI rank and _logfile.txt to the name provided. E3SM passes EAM a log file name to use for logging messages, so it would be nice to have a mechanism for specifying exactly that log file.

Describe the solution you'd like
Can we add a constructor to the Logger class that was able to accept a given log file name? I don't know how all of this fits in with MPI ranks and such. I think we should draw @AaronDonahue into this conversation.

[bartgol]

I completely agree. Instead of adding _logfile.txt we could simply add .log, since that's a widely recognized extension for log files.

As for the MPI ranks, we can also provide an optional ctor arg, that tells whether we should add rank info in the log file.

[bartgol]

Uhm, I am looking at the impl, and noticed that "rank0-only" is an optiion settable after construction. Considering that such action (namely, turning off logging for rank>0) cannot be undone, it could easily be done at construction time. Even better, we could encode it with an extra template argument (which, I think, was the original design).