Source Injection functionality
Added signal injection method for SimulateSpectrogram class. Also added arg to get_spectrogram to permit not injecting a signal, i.e. return just noise spectrogram (injects signal by default)
Added signal injection method for SimulateSpectrogram class. Also added arg to get_spectrogram to permit not injecting a signal, i.e. return just noise spectrogram (injects signal by default)