Adjust the stepsize to keep the acceptance ratio at a good value
Notes
when ratio naccept/nsteps < target_ratio then decrease step size by a correction factor f (new-step-size = old-step-size*f, where 0<f<1), else if naccept/nsteps > target_ratio, then increases the step size (new-step-size = old-step-size/f, where 0<f<1), although this is increased with an upper bound: max_stepsize.