irit-rst-dt evaluate --resume

Pick up an evaluation where we left off
irit-rst-dt clean: remove intermediary eval files

Kill:
- any scratch directories
- any incomplete eval dirs

It's also tempting to get rid of feature directories without
evaluations but it's not likely to be a common case in future
development
Flesh out harness documentation
Say more in intermediary banners

I want it to be as easy to know where we are at a glance.
Use attelo harness symlink util