A few GEOS functions that could enhance sfnetworks

[agila5]

Is your feature request related to a problem? Please describe.
Hi! I was skimming through the GEOS docs and I noticed a few functions that may be useful for this package. I will list them here. I'm not sure (if or) how these functions can be added (maybe extend the sf package or check the approach documented here).

btw: I have little to no experience in these cases, so feel free to ignore here if the following suggestions do not make sense.

Describe the solution you'd like

1. GEOSNode(): For linear inputs, returns a new geometry in which no lines cross each other, and all touching occurs at end points. Also related to Enable edge subdivision at every edge crossing #134
2. GEOSLineMerge(): Sews together a set of fully noded LineStrings removing any cardinality 2 nodes in the linework. I think it might be related to the "to_spatial_smooth" morpher.
3. GEOSGeom_extractUniquePoints(): Return all distinct vertices of input geometry as a MultiPoint. Note that only 2 dimensions of the vertices are considered when testing for equality. Might be related to the building of sfnetwork objects and some morphers;

Other tools that may be useful:

1. GEOSDensify(): add new nodes at a fixed distance;

Describe alternatives you've considered
Ignore everything here? 😅