ctf.core: introducing ICTFPacketDescriptor

ctf.core: introducing ICTFPacketDescriptor

This is the interface of the "StreamInputPacketIndexEntry".
The documentation is also slightly expanded.

Change-Id: Ic02762ab90c0bf69766c7bcf90ddd2207e7e12f6
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/45929
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>