Support relational OSM river definition

[rosepearson]

Currently only support way type OSM rivers for defining alignment

Support relational ones too. This is relevant for places like Whakatane Mouth: https://www.openstreetmap.org/way/970645729 Upstream: https://www.openstreetmap.org/way/926635708 combined: https://www.openstreetmap.org/relation/13389298

Note that way/relation ids are not unique across object types so safest to get the user to specify 'relation' or 'way'.

Another option is supporting a list of ways and stringing them together in order. For instance in Waikanae downstream: https://www.openstreetmap.org/way/132793862 midstream: https://www.openstreetmap.org/way/1128615001 upstream: https://www.openstreetmap.org/way/1131530569