Removing a vertex from an UndirectedSparseGraph is very slow, make it faster

[Dani]

Is your feature request related to a problem? Please describe.
In an application, I have to create an UndirectedSparseGraph from which I then filter out (i.e. RemoveVertex) many nodes, which takes minutes for a graph of less than a thousand vertices.

Describe the solution you'd like
Optimally, this should be taking seconds, instead of minutes. I've created a pull request that achieves this: #130

[github-actions]

Thanks for supporting the development of C# Algorithms with your first issue! We look forward to handling it.