nurse shark, first draft

Open for feedback, but *not* for merge. I'll be working on this branch as well to update the viz code for the new data model.

The only things you can test here presently are validation with the node script `dev/validate-with-timing.js` and the tests for nurse shark, `test/nurseshark_test.js`. The visualization itself is broken, as expected.

@cheddar I added a couple of things that were missing to the schematron, as per my comment on the Trello card.
@kentquirk I thought we had agreed never to rewrite history in public repos?

Otherwise @cheddar you are welcome to create a Trello card for this as a potential item for sprint. It won't be hard to switch to the blip-mock-data repo (although maybe it should be renamed to reflect being used for things outside of blip). I myself don't understand the urgency. I think we can continue with checking in the demo data only when necessary; the damage has already been done in terms of repo size.