Day 21 Part 9 - GraphEdgesLabelled, prepare support for Digraph

In the constructor of GraphEdgesLabelled, we add an additional parameter
'isDirected' to indicate whether the graph is directed. This is to pave the
way for this class to support the Digraph class for the `findCycles` method,
since the cycle finding algorithm remains the same.

Signed-off-by: Pang Yan Han <pangyanhan@gmail.com>
Day 21 Part 10 - Complete P82 Digraph.findCycles

We needed to alter the constructor of GraphEdgeLabelled to take in a
GraphBase instead of a Graph so that we can pass a Digraph into it.

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