Draft: Audio overhaul

Overhaul of the audio module.

Main objectives:

• Cleanup and simplify code base to make code maintenance easier
  • Remove support for "stacked" audio objects
• Standardization naming conventions with the rest of Ketos code base (#4)
• Resolve situation around supported audio file formats (#16)
• Improvements to waveform/spectrogram interfaces, esp. related to instantiating new objects (#2)
• Replace existing Numpy/Scipy algorithms for spectrogram computation with new Tensorflow algorithms (!10)
• etc.