Added options to bypass ffmpeg usage in some cases ; Pyro client now uses argparse

[valentinp72]

• When the source audio files are already wav/flac files sampled at 16 kHz, it is useless to call ffmpeg and then reload that with soundfile. Now, when ffmpeg is None, the audio files are loaded directly with soundfile.
• I added argparse to the Pyro client for parameters such as URI, batch size and ffmpeg.