Define RiverNetwork vs Runoff Generation Nexus

[mikejohnson51]

Right now, we can identify nexuses on an aggregated network using get_nexus_locations . We also need to be able to identify routeing "outflow" nexuses on the original refactored, non-aggregated, flow path network.

Look at using upper/lower catchment associations: https://opengeospatial.github.io/HY_Features/HTML_HYF/HY_HydroFeature/index.htm?goto=2:42

@dblodgett-usgs; @hellkite500