Add absolute rms noise check

This MR addresses RAP-473 by adding a check of the absolute value of the noise to the selfcal convergence/divergence check. A new parameter, called failure_ratio, has been added to the strategy to set the maximum allowed ratio of (median noise) / (theoretical noise). If this ratio exceeds that set by failure_ratio, selfcal is stopped (and the final cycle, if any, will be skipped).