The copyright and author stuff is not necessary, out-of-date,
and inconsistent.  It takes up visual space and is a pain to
police everyone doing the same thing on the top of the module.

Git handles authorship in a comprehensive and authoritative way.
The LICENSE.txt file applies to all project code.

Found via `codespell -q 3 -I ../networkx-whitelist.txt` where whitelist consisted of:
```
ans
childs
iff
nd
te
```

Change double `` to single ` for all function arguments.
Remove double `` around True, False, None
Leave double `` when a literal python expression is intended.
I found a couple of places where math mode was intended.
Still need to look for those.

This commit changes the behavior of the `closeness_vitality()` function
so that it only computes the Wiener index of the original graph once
instead of recomputing the same value for each node in the graph.

Previously, the closeness vitality function was using an incorrect
definition of the Wiener index, counting each orientation of a pair of
nodes in an undirected graph twice. This commit corrects the behavior of
the Wiener index function and therefore the closeness vitality function.

Correcting the Wiener index function also corrects another case for the
closeness vitality function: when removing a node causes a graph to
become disconnected. By definition, the closeness vitality of a node
whose removal causes the graph to become disconnected should be negative
infinity. This commit fixes the Wiener index function to return positive
infinity on a disconnected graph.

Furthermore, this commit improves the closeness vitality function so
that it does not mutate the graph in order to compute the vitality
value.

Finally, this places the Wiener index function in its own module,
documents it, and makes it available to the end user.