Enabling Support for Dynamic Graphs: Allow adding and removing vertices

[larissapoghosyan]

After #225 graph refactoring this change simply enables adding and removing vertices to dynamic graphs.
Addresses the feature request #226.

• Added add_vertex implementation. This was trivial
• Added remove_vertex implementation - This required careful cleanup of existing edges, matrix rows and columns, in reverse order.
• Added simple tests covering addition and removal of vertices, including scenarios when we need to remove a vertix which has edges.