Add test_shortest_path_simple() to test_shortest_path.py to test finding the shortest path with dijkstra on a very simple graph
Add shortest_path.py with first working version of dijkstra()
Revise dijkstra() to update the dictionary of previous nodes only when the distance to the next node is less than the current total distance to that node
Add test_shortest_path_triangle_two_edge_path() to assert dijkstra() determines the shortest path even if it takes multiple edges