Expose strahler stream network in delineateit

[phargogh]

Rafa just stopped by to request if we could add a stream network creation step to DelineateIt, since it would be useful for a couple of upcoming projects. Rich had added a Strahler Stream Network delineation function to pygeoprocessing, and Rafa says that this is sufficient for what he's thinking about.

The definition of what it means to be a "stream" gets a little murky for MFD, so since DelineateIt allows both D8 and MFD, this optional output of DelineateIt would only be available if the user has selected D8 routing.

[phargogh]

Let's bump this to a future release.

Explaining the various statistical inputs to this so that we don't potentially get a bunch of forums questions about it will take a little time that I don't have right now and I don't think this should hold up the release.

I've started working on this a little bit on my InVEST fork: https://github.com/phargogh/invest/tree/feature/349-expose-subwatershed-delineation-and-strahler-streams. Subwatershed delineation has not yet been added.

One other thing is that this branch is adding these changes to RouteDEM rather than DelineateIt. The reason for this is that DelineateIt is clearly focused around delineating watersheds by user-provided outflow features. By contrast, RouteDEM seems a more natural home for this because everything within RouteDEM is derived from the complete landscape, plus slope (which isn't strictly routing) so it's a bit more of a hodgepodge of routing-related functions.

[phargogh]

This work is being done as a part of #349