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

**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: https://github.com/aalhour/C-Sharp-Algorithms/pull/130/