Convert a set of line segment "join decisions" into a weighted adjacency matrix describing which map unit symbols touch.
joinAdjacency(x, vars = c("l_musym", "r_musym"))data.frame or similar object, each row represents a single shared edge (typically sf LINESTRING feature)
a vector of two characters naming columns containing "left", and "right" map unit symbols
A weighted adjacency matrix is returned, suitable for plotting directly with plotSoilRelationGraph().