Only disambiguate mainstem river when using drainage area.

[dblodgett-usgs]

In some cases, when drainage area is used to disambiguate which flowline a gage is on, a gage is snapped upstream of a tributary because the drainage area at the bottom of the flowline that includes the tributary is far away and a much larger drainage area.

In reality, the gage is just downstream of the tributary and we aren't considering the catchment split drainage area in the disambiguation.

The fix here is to use drainage area to find the correct main-path then snap to the nearest location along that main path.

There will need to be some override -- where if the nearest point is downstream of a tributary is way off in terms of drainage area we don't use it. Will be interesting to see how many places this changes things.