Solver stopping

Add parset key 'stepsigma' to enable early stopping of the solver, comparing mean_step_size < stepsigma x std_step_size, if mean_step_size goes lower than the limit on the right hand side, iterations are stopped because no possible improvement can be expected (limited by inherent noise). A large value ~0.5-1.0 can be used in early major cycles because the sky model is not complete to aim for a refined solution and can be set to a low value ~0.1 at the final major cycles with the full sky model.