Added non-deterministic lists, trees and buildTree method

[Liya-F]

• Inside the ND-experimental folder, I added three files ND-trees, ND-lists, and ND-buildTree
• ND-lists contains non-deterministic list representation and methods over the list
• ND-trees contains non-deterministic tree representation and methods over it
• ND-buildTree contains a method called buildTree and it's helper functions which builds a non-deterministic tree from an expression