Added Gell-Mann matrices

Improve the energy evaluation. Add the equivalent expression of 2-site permutation using the quadratic Casimir operator of su(3).

Express 2-site permutation in terms of su3 generators.

Spin-1 BBH model with next nearest neighbor interactions on the square lattice. Add the feature of combing a base Hamiltonian (self designed) and a target Hamiltonian (true Hamiltonian) together.

To be consistent with kagome.py, trace1x1_kagome() -> trace1x1_dn_kagome()

Fixed the bug in energy_1site function

Add the method boundary_spectrum(n_site=2, top_n=27) which calculates the spectrum of the edge that is built by the environmental tensor T.

Update boundary_spectrum(env, n_site, top_n): env - environment tensors; n_site - number of T tensors; top_n - number of leading eigenvalues to compute

Calculate correlation functions along  (0, 1) (0th column) and (1, 0) (1st column) directions and the transfer operator spectrum (in order to get the correlation lengths)