[8pt] Levelpath outlets terminate outside of HUC before reaching WBD buffer

[mluck]

The levelpath of the outlet(s) of a HUC may terminate outside of the HUC before it reaches the edge of the DEM, resulting in reverse-flow and faulty networks during pit-filling. This is a similar issue to #1280.

Current behavior

The light blue (cyan) levelpath ends just outside of the HUC (black line) and doesn't extend to reach the edge of the WBD-buffered DEM (green line). This causes the DEM-derived reaches (red line) to deviate from the NWM streams, and drops segments (red dashed lines) and catchments (shaded areas) of the DEM-derived reaches where the crosswalk snap distance (100 meters) is exceeded.

Expected behavior

The yellow NWM stream segment should be added to the outlet levelpath.