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.
Oops, yeah, that's what I meant but it's not very clear the way I wrote it; updated so it's clearer.