Transcoder/StreamTranscoder: rename init to preProcessCodecLatency

* init does not mean anything.
* pyTest testTranscoderDummy: remove uncessary call of
preprocessCodecLatency.
Transcoder: fix preProcessCodecLatency

Need to call it after beginWrap.
OutputFile: check if at least one stream exists when getProgressDuration
Merge pull request #16 from cchampet/init_to_preprocessCodecLatency

Init to preProcessCodecLatency