Add a `local_storage` package that finds a suitable place to put files.
Currently it just looks at the location of the currently running
executable; it can later do other things. It can be queried for directory
to put job-specific files.
It is intended to be used by the under-development "last rendered output"
processing system, to store an image file per job. Later we should also
refactor the task log handling system to use this.
