Provenance on steroids

[smondet]

Still to do:

• [x] use the Provenance_description.t to output a .json (and .csv?), to saved (or all?) files.
• [x] propagate the change to make sure extensions of the EDSL are somewhat easy to update.

This has already the advantage of keep around the JSON serialization of the configurations (and not just their name).

[smondet]

@armish this is ready for review, cf. also https://github.com/hammerlab/epidisco/pull/196