VideoProperties: expose bitDepth when getPropertiesAsMap
VideoProperties: avoid copy of gopStructure when getPropertiesAsMap
VideoFrame: rename getPixelDesc to getPixel

* Get a reference to a Pixel.
* Give a reference to a Rational when setDar.
frame: clean define in headers

Also clean comments of struct ColorProperties in VideoFrame.
InputFile / OutputFile: update doc of constructor
InputFile: seek at the beginning of the file only if level > LevelHeader
Pixel: remove default constructor
VideoProperties: add getPixel
Pixel is a wrapper around AVPixelFormat and AVPixFmtDescriptor

* VideoProperties: move functions about pixel in Pixel class.
* Expose pixel informations when getPropertiesAsMap.
* Update app avplay to correctly create a pixel.
* Update pyTest testSetFrame to correctly create a pixel.