Incorrect number of pairs and doc error in selfCoordNum

[giacomofiorin]

Describe the bug
The number of pairs in selfCoordNum is calculated from a wrong formula:
$N_{\mathrm{pairs}} = (N-1)(N-1)$
However, it should be:
$N_{\mathrm{pairs}} = N(N-1)/2$
(as correctly reported in the doc).

Additionally, the doc wrongly lists cutoff3 as an allowed keyword.

This expression is used in two places:

1. To defined a default upperBoundary for the selfCoordNum function, which would only apply if the user doesn't provide any.
2. To allocate a pairlist buffer, but this is harmless because it simply results in a bigger buffer than needed.

Back-end (i.e. package that uses Colvars)
All

Colvars version
All since 020f555

Expected behavior
The number of pairs should be calculated correctly

Output log
Not message in the log: the number of pairs is used to define default boundaries for the CV internally, which would only apply if the user doesn't provide any.