Fix inconsistent index and count when removing vertex from Graph.

[AngLv]

DirectedWeightedGraph.RemoveVertex decrements the Count property, but keeps the index of vertex array and adjacency matrix. The inconsistent index and count cause issue when adding new vertex after removing vertex.

This PR resolves the issue and fixes #368.

• I have performed a self-review of my code
• My code follows the style guidelines of this project
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Comments in areas I changed are up to date
• I have added comments to hard-to-understand areas of my code
• I have made corresponding changes to the README.md

[siriak]

Looks good, thanks!