Add nexus realizations for frontal and inland catchment outlets.

[dblodgett-usgs]

Per https://docs.ogc.org/per/22-040.html#_frontal_and_inland_sink_nexus_types we should consider adding nexus realizations for both frontal outlets -- where a catchment can be defined along a shoreline an thought to drain to a diffuse outlet or terminate in an internal diffuse outlet like a playa or a salt lake (bed).