multi: add base dir to aperture

[carlaKC]

This PR adds a base dir to aperture for move convenient file placement.

There's always some thinking to so with our various file placement flags (specifically --configfile and --basedir), gone with the following logic:

• Default to AppDir if nothing is set
• If only --configfile is set, use AppDir for files, use custom config location and require that it exists
• If only --basedir is set, look for aperture.yaml in basedir but do not require it
• If --configfile and --basedir are set, use basedir for files and require --configfile exists

Reason for allowing a --configfile and --basedir is that custom config file allows you to set the name of your config file as well as location.

[carlaKC]

@guggero @ellemouton PTAL 🙏