Add frequency response analysis: gain_margin, phase_margin, frequency_response, coherence

[Kotakku]

baglab.analysis — additional functions

Function	Purpose	Priority
gain_margin(cmd, response, fs)	Gain margin	Medium
phase_margin(cmd, response, fs)	Phase margin	Medium
frequency_response(cmd, response, fs)	Bode plot data from measured data	Medium
coherence(x, y, fs)	Coherence (input-output linearity evaluation)	Low