Expose the Duration Field for Frames

[Queuecumber]

This PR exposes the duration field on frame objects as the time that a given frame should be displayed for in time_base units.

This can simplify certain decode loops which are searching for specific frames in time, e.g.:

for frame in container.decode(video=0):
  if (frame.pts <= target_time_pts <= frame.pts + frame.duration):
    break # Found it

In this example the frame might not occur at the target pts so we need to use duration to check if the frame would be onscreen at the target time.

Currently this can be done by computing an average frame duration using the average frame rate (which is error prone) or trying to synchronize to the packets duration field.

[Queuecumber]

Thanks for the fast turnaround on this