Sngl src shortest path for unweighted graphs added

Implemented only for undirected graphs.
Test module for operatns on simple digraphs added.
Module for simple digraph operations added.

Unary and Binary operations included.
traversal.py module changed to graph_traversal.py

Only traversal algorithms for simple undirected graphs is in this module.
useless mem  allocation removed in graph_traversal

previously arrays were uselessly allocated with larger memory spaces.
Module implementing digraph traversals added.
Traversal modules modified to return smaller array

Earlier space was allocated even for the vertices not traversed at all.
Docstrings of functions made more informative.

modified for traversal and shortest_path functions.