delineate_watersheds_d8 does not handle the case where SRS is undefined

[phargogh]

D8 watershed delineation implicitly requires that the flow direction raster has a defined spatial reference and doesn't handle the case where the SRS might not be defined. The projection is not actually used in the delineation, it's only assigned to the output datasets. So, we should handle this case and allow watershed delineation to proceed or throw an error and make this really explicit.

We should also make sure that the flow direction raster and outflow vector have the same projections or that both have no projection.