single-particle Green's function with Lanczos method

[j-otsuki]

I would like to compute the single-particle Green's function and the single-particle excitation spectrum with the Lanczos method. I hope this functionality will be implemented in the future release.

It would be nice if we can compute G_{ij}(z) for arbitrary spin/orbital/site index i, j and the (imagiary) frequency z. Here, pure imaginary z corresponds to the Matsubara Green's function and real z (with infinitesimal imaginary number) corresponds to the retarded Green's function.