Input & Output file format

[ZigRazor]

Discussed in #170

^{Originally posted by sidml November 2, 2021}
I was thinking that we should ensure compatibility with existing popular formats for saving and loading graphs. Following is taken from here

The BGL currently provides two graph classes and an edge list adaptor:
adjacency_list
adjacency_matrix
edge_list

The adjacency_list class is the general purpose “swiss army knife” of graph classes. It is highly parameterized so that it can be optimized for different situations: the graph is directed or undirected, allow or disallow parallel edges, efficient access to just the out-edges or also to the in-edges, fast vertex insertion and removal at the cost of extra space overhead, etc.

The adjacency_matrix class stores edges in a |V| x |V| matrix (where |V| is the number of vertices). The elements of this matrix represent edges in the graph. Adjacency matrix representations are especially suitable for very dense graphs, i.e., those where the number of edges approaches |V|2.

The edge_list class is an adaptor that takes any kind of edge iterator and implements an Edge List Graph.

Let me know what you guys think.
@ZigRazor @AlfredCP @pavan-pan