Day 21 Part 5 - Complete Digraph.fromStringLabel

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 21 Part 6 - add given test case for Graph.fromString

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 21 Part 7 - Correct edge for Digraph from '->' to '>'

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 21 Part 5 - Complete GraphBase.toAdjacentForm

Thereby completing P80

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 21 Part 6 - Complete P81

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 21 Part 7 - Modify Graph::addEdge to add only one edge for self loop

The original Graph::addEdge would have added 2 edges to the adjacency list of
the vertex involved in the self loop, whereas now, we add an additional check
so that only 1 edge is added on a self loop.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>