Feature/scenario create

See commit comments for details, probably best to view diff just for https://github.com/WorldBank-Transport/open-transit-indicators/commit/4d5f26094528be672e93e0c0cb9cd5ab5c6dc6b1 since that's where all the meat is.

Soliciting feedback on the new model structure, should be a bit more flexible and isolated.
Good point. The organization I chose was a compromise due to the way the routes are automatically instantiated in app.js. I think your second suggestion is the best, but it would require refactoring of the routes code in app.js that I didn't really want to mess with.

The first, I'm not so sure about, as these views are all subsets of scenario, and require a scenario to work properly.