Add iterative solvers for exact steadystate of open system

Created by: PhilipVinc

Uses a trickUses a trick from my PhD thesis to find the steadystate through bicgstabl.

Essentially iteratively solves the linear system L\rho = 0 Trace[\rho] = 1

In principle, in compiled languages I was able to push the method to handle up to 13-14 spins, but numpy does not seem to handle more than 10-11...

Still, it's much better than the diagonalization approach which cannot be used for more than 6-7 spins.