Add Monte Carlo sweeps for equilibration before making measurement
Created by: wuyukai
In ground_state.hpp the sampling of the spin configurations directly starts from an initial configuration, which is random at iteration 0. Therefore at this step the sampled derivatives can have large error as the Monte Carlo sampling has not reached equilibrium for the first few sweeps. This is not a big problem for large number of iterations; but if I want to start the iteration from a previous one by specifying "InitFile", even the first few iterations become important. So I suggest to add some sweeps, say, 1/10 of the total number of sweeps, after setting random initial spin configurations. This will not significantly change the timing because it is not needed for the later iterations.