SNR and THD measurements

any plans to add utils to add SNR, THD, THD+N measurements for conducted measurements, i.e. line in/out signals?

Two pull requests cover this. #64 adds pytta.sine_distortion, which gives THD and THD+N of a recorded sine, and pytta.snr, which gives the SNR from a signal recording and a noise recording. #57 adds pytta.harmonic_distortion, which gives the THD over frequency from a sweep measurement.