StreamTranscoder: add a more explicit messages when verbose
Transcoder: rename getStream to getStreamTranscoder
pyTest: add a blanck line at the end of files
AvInputStream: remove getPacketDuration

* The functions returns duration of last packet added.
* Not what we expected, and not used.
Transcoder: remove _outputFps attribute

* The method setOutputFps does not do what the user expected: the fps of
Transcoder is only used for progress bar (no consequencies in transcode
process !).
* This kind of method must be implemented in the outputStream on which
we want to update the fps.
AvInputAudio: rename getNextFrame to decodeNextFrame

This private function decodes the next frame (call
avcodec_decode_audio4).
AvInputVideo: check decodedSize

* Return false as status of decoding if size of frame is 0.
StreamTranscoder: clean documentation of attributes
AvInputVideo / Audio: add documentation for attributes
AvInputVideo / Audio: fix _selectedStream
AvInputVideo / Audio: has a link to the video / audio codec (decoder)
AvInputVideo: refactore readNextFrame

Symmetry with AvInputAudio: add private function decodeNextFrame.
AvInputStream: add documentation for attributes
AvOutputStream: add documentation for attributes
Generator Video / Audio: remove _codec

Get Video / Audio FrameDesc from a frameDesc instead of a link to a
codec.