Standardize terminology and add it to vignette

[luukvdmeer]

In graph theory there does not really seem to be a standard way of naming "things". For example, you can talk about vertices or nodes, edges or links, leafs or terminals, incident or adjacent, boundaries or endpoints, etc etc. Even within the package (e.g. in documentation but also in internal variable naming) we sometimes use different naming for the same thing. I think it would be good to standardize this and add a "terminology" section to the first vignette in which we describe the terminology we use in sfnetworks.