Resolve "Marker between trial 1 and trial 2"

Resolves #28 (closed). I broke actions out from definitions.py to their own files in config. That's right, files. I also separated actions that change the block assembly from more abstract "tags", like "touched reference model" or "first touch", and implemented the marker between trial 1 and trial 2 as another tag.

Finally, compute_stats.py outputs another file in data-export: tags.csv, which contains the ID and start/end times of any annotated tags.