Optimize Python Problem 14

Running from MAX to 2 is faster than 2 to MAX.
Add Python Problem 15
Create euler.graphs with PyramidGraph

PyramidGraph stores a graph that is pyramidal in shape. It computes the
largest sum of any path from the top to the bottom in an efficient
manner.
Add Python Problems 18 and 67

These use the euler.graphs.PyramidGraphs class to quickly compute the
maximum sum of a path from the top of the pyramid to the bottom.