... when it is detected that there may be numerical inaccuracy in dt polynomial calculation.

• this is intended to resolve numerical issue observed in DT margin calculation when there is a large difference between the size of numerator and denominator coefficients stability_margins doesn't find gain=1 crossover frequency correctly for DT systems #523. I am not sure how good the method is I used to detect this issue is any good. In fact I am sure it is not. comments welcome here.
• can use method='poly' to override and force polynomial evaluation, or method='FRD' to force to using interpolated FRD to calculate gain and phase crossover frequencies.