kontrol.regulator.feedback.mode_decomposition() doesn't work well with relative order higher than two

[terrencetec]

It gives negative gain and gain peak because of the phase.
It messes up kontrol.regulator.post_filter.notch() when it trys to compare peak gain and the static gain

kontrol/kontrol/regulator/post_filter.py

Lines 359 to 366 in e95169e

	gain_peak = q*k # magnitude at the resonance
	# Choose modes that have peak higher than the DC gain.
	gain_mask = gain_peak > k
	fn = fn[gain_mask]
	q = q[gain_mask]
	k = k[gain_mask]
	gain_peak = gain_peak[gain_mask]

As a temporary solution, I changed

    gain_mask = gain_peak > k

to

    gain_mask = abs(gain_peak) > abs(k)