Enable switching of outfall detection method

[barneydobson]

Right, what you have here can be an option that users can activate if they really need multiple possible outfall locations with the caveat that it only considers proximity to water bodies. Having said that, I think adding more complexity to this outfall detection can be a low priority for the paper, since I think we already have enough material that we don't need to focus on this part, and we can leave it as a future direction.

Originally posted by @cheginit in #144 (comment)

Current method uses a mix of OSM rivers and lowest elevation point. Conceptually it may be more sound to just use lowest elevation point. Or certainly a user may want to choose.

This would be straightfoward to add a parameterised switch for.