Factor system

Split the amplitude into different components by the factor of each sum rules.

A = a1 ( B x C x D) + a2 (E x F)
B = b1 B1 + b2 B2

is a tree structure

A -> [(a1, [(b1, B1), (b2, B2)], C, D), (a2, E, F)]

Each component is a path for root to a leaf.

(a1, b1),  (a1, b2),  (a2,)

This pull request will implement the function for creating the combination and calculating each part of amplitude.