simplifybyselectsite: Simplify the river topo (arc.set) with sites along the river
In WeiquanLuo/rivertopo: What the Package Does (One Line, Title Case)

Simplify the river topo (arc.set) with sites along the river

1	simplifybyselectsite(site_sf, arc_sf.set, mouthID)

`site_sf`	a site sf object with column: id, lon, lat, geometry, X, Y, and the snapped point information from the riverdist::xy2segvert() columns: seg, vert, snapdist
`arc_sf.set`	a arc sf object from makearc() containing columns: from, seg0, vert0, x0, y0, to, seg1, vert1, x1, y1, geometry
`mouthID`	a string of the site id as the mouth point appear in site_sf