Added factor decomposition, which uses the SVD to factorize a tensor into just two factors by multiplying the square root of the singular values into the U and V matrices. Added helpful functions which return vectors of "bonds" for common two-dimensional lattices. These are extremely useful for making 2D Hamiltonians as MPOs using AutoMPO. See the example here: http://itensor.org/docs.cgi?page=formulas/2d_dmrg