spin-orbital 2-particle density matrices

This need is firstly opened in pyscf issue https://github.com/pyscf/pyscf/issues/357 . 2-particle spin-orbital density matrices  <i_alpha^+ j_alpha^+ k_alpha l_alpha>, <i_alpha^+ j_beta^+ k_beta l_alpha>, <i_beta^+ j_beta^+ k_beta l_beta> are needed to analyze spin correlation function <S1 dot S2>