Current Elem::set_node does not work well for C0polyhedron

[GiudGiud]

It does not update the sidelinks_data, e.g. all the polygon faces's nodes.
This has consequences, the node isn't really "changed" depending on which routines you'd ask

[roystgnr]

Wow, this is fascinatingly embarrassing. Ben made set_node() virtual in 2004 in preparation for a Side subclass, he didn't ever end up needing to override it, and then it just stayed virtual forever, despite no overrides except for a thoughtless use of one (if I'd thought then I'd just have made it non-virtual then) to add an error message in RemoteElem.

Even if set_node hadn't already been virtual I'd say this was worth making it virtual since we don't call it often enough to notice the overhead, but as-is this will be "free" to fix.

We'll probably want to get rid of the deprecated just-returns-a-reference overload while we're at it.

[GiudGiud]

will do, thanks for the guidance!