Problem 15 - WIP

This implements the down and right lattice path tracing as a
recursive function call, summing the path count at the leaves.
It is fast enough for up to about 15 vertices but fails at 20
due to its exponential run-time.
Cleanup
Problem 15 - WIP

Take advantage of the pattern: the sequence is given by the
following formula (where C is the function n-choose-k)
    Number Of Paths = C(2 * VertexCount, VertexCount)

This method using the additive recursive formula also is too
slow for 20 vertices.
Problem 15