Flush directory prompt, explicit path full integration, and external subpop directory added

*On directory flush, user is now prompted to flush, keep, or backup to
a local copy directory.
*Explicit path now also affects file output. Study name folder will be
created inside the explicit directory given.
*External subpopulation folder support added. Chopper pulls from gdoc
provided directory and AV diagnosis based interventions point to said
directory.