valley vignette

[fnattino]

We have no vignette on the valley delineation, and we currently provide little information about the procedure used to draw valley boundaries (apart the vignette, we should have some description/links in the valley.R module).

The methodology should reference this paper (see original issue on valley delineation: CityRiverSpaces/prototyping#1), but we should also have a look at the paper(s) suggested by its reviewers (see "Discussion" tab at the journal link above: was the methodology already presented earlier?)