Encoder: Mp3Encoder add xingmux in the pipe in order to fix duration

From Gstreamer manual :
"xingmux adds a Xing header to MP3 files. This contains information about the duration and size of the file and a seek table and is very useful for getting an almost correct duration and better seeking on VBR MP3 files."
Encoder: fix calling arguments for m4a
Encoder: WebM -> Suppress the apparently useless element 'mux.' in the pipe
Encoder: Enable timestamp during encoding, provide timestamp and duration in tools/numpy_array_to_gst_buffer.py
Decoder: utils -> Fix indentation and division issue
Analyzer: WaveForm -> add pre-processing decorators
Tests: Various enhancements and fixes in transcoding, encoding

- unit-timeside: Small enhancements in Error messages display
- unit-timeside: Remove 'assertSameList' function which seems equivalent to unittest assertItemsEqual. unittest.TestCase class does thus no more need to be override, propagate this change in every test_* modules.
- test_encoding and test_transcoding: refactoring + handling of expected failures (duration for loseless encoders, channels for voaacenc, ...) + run everything in pipes to handle properly the setup.
mp4mux for the aac encoder