Add explicit Graph, DiGraph, MultiGraph & MultiDiGraph types

[Azer0s]

Right now, there's only a single Graph type. This needs to be refactored into these 4, NetworkX like, graph types to efficiently implement all algorithms. Seems like a good place for an abstract class.

[rudreshr17]

Hi! I stopped working on BFS issue due to my college assignments. Anyways, you mentioned in that issue that this is of more importance, hence I would like to try this first. ? But I don't understand what you meant by this:

Seems like a good place for an abstract class.

As I understand refactoring means optimizing the current graph code and creating different code for digraph,mulitgraph etc.

I'm going to go through the code for the graph data type that you have created and try and understand it, the I'm going to try and refer some sources so as to do the same for digraph etc. If there's anything more you can guide me on, please do, Thanks and sorry for the long message.