Enh/spectogram/add mel
This merge changes the sound_classification.spectrogram module. I made the Spectrogram into a parent class that is then subclassed to make the MagSpectrogram, PowerSpectrogram and MelSpectrogram subclasses (for magnitude, power and mel spectrograms, respectively).
These classes take an instance of the AudioSignal class as input (as opposed to an image, as the original Spectrogram). I updated the tests changed, although more tests could be useful.
I also changed the audio fixture in conftest.py to sine_audio, so tests that use that fixture need to be updated.