This will convert 2 objects whose coordinates can be extracted with
sp::SpatialPoints*) to a single
The first object is treated as the "to" (destination), and the second object the "from" (source).
This can be used to represent directional
SpatialLines, especially with with arrow heads,
Plot(sl, length = 0.1).
SpatialPoints* object. Optional. If not provided, then the function will
attempt to find the "previous" coordinates as columns (