Add VertexConnectivity

[ffloresbrito]

This pull request adds an attribute called VertexConnectivity which for a graph G = (V, E) computes the least cardinality |S| of a subset S of V, such that G - S is either disconnected, is trivial, or has no vertices. Documentation and tests are included.

[wilfwilson]

A few more fairly minor comments. I've still not studied the algorithm in detail, to see whether you're implementing it correctly. Not sure whether I will. Maybe @james-d-mitchell knows more about the algorithm and could weigh in?

[james-d-mitchell]

This looks quite reasonable, apart from some design decisions that are likely rather costly (in terms of time and space). I'd like to see the code in this PR refactored to remove these things.

[wilfwilson]

@james-d-mitchell Did @ffloresbrito address your changes? Is @ffloresbrito still involved in any GAP-related activity?

[wilfwilson]

@james-d-mitchell I have squashed and rebased this PR on the latest master branch, and fixed the CI errors, and pushed. Assuming that @ffloresbrito is no longer involved with this, we can now decide how to proceed with this PR (or whether just to just close it).

On the whole, I'd say it's better to have a slow implementation than no implementation - it could always be a student project to improve or rewrite it. But that assumes that this implementation is correct, and it's been long enough that I don't remember if it is. So I'll review it again sometime.

[wilfwilson]

I accidentally marked a comment from James as resolved, but it's not. It basically said:

You should add a citation to the documentation, stating where this algorithm is taken from.

Looks like all of the specific changes that James asked for were made.

[wilfwilson]

I've gone through the specific changes that James and I requested, and everything (bar the comment above) was resolved. So I've marked the comments as such.

I think we can assume that @ffloresbrito is no longer involved with this PR. I'm going to give it a proper review, and if it requires only small changes to make it acceptable, then I'll do this and get the PR merged.

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