FEATURE: put detail files in a subdirectory (optionally?)

It would be nice to be able to put all of the detail files in a subdirectory to make it easier to see what is where in a report directory. This is particularly useful when e.g. keeping multiple runs with timestamped file names.

The subdirectory would have the same name as the index file, minus the suffix (e.g. File.rootname/1).

This could be controlled with a boolean option such as scenarios_in_subdir.