Spectrogram class and AudioSignal namedtuple
This merge request introduces two new things:
- an AudioSignal namedtuple
- a Spectrogram class
thereby resolving Issue data_analytics_dal/packages/sound_classification#14.
The AudioSignal namedtuple simply combines signal (numpy array) and rate (float) in a single object.
The Spectrogram class provides stores information such as frequency resolution, frequency range, time resolution, etc and additionally provides useful methods such as cropping along the frequency axis and computing averages and medians for selected frequency bands.
Unit tests have implemented to check most of the newly implemented methods.