Having a prefix for log dir

[matham]

Given that log_dir could be non-empty, we should have a prefix that can be made unique in case the file already exists. Or even better, have a default date/time format string so that by default, if not empty, will make it unique. Because otherwise, the h5 files may accidentally write multiple runs to the same file.

Even if users typically handle it, it'd be nice to have some nice defaults if users don't use the simple pipeline.

Originally posted by @matham in #1 (comment)