Adding support for reseting VideoReaders

[brimoor]

All VideoReaders now implement a reset() method, which allows them to be reset at any time such that the next frame read will the first frame.

This addresses the second half of #373, which has been updated to reflect that the only remaining need is an efficient implementation of seeking to a specific frame.

Test code:

import eta.core.video as etav

video_path = "/path/to/video.mp4"
frames_dir = "/path/to/video/frames"

max_frames = [10, 20, 15]

video_reader = etav.FFmpegVideoReader(video_path)
#video_reader = etav.OpenCVVideoReader(video_path)
#video_reader = etav.SampledFramesVideoReader(frames_dir)

with video_reader as vr:
    for max_frame in max_frames:
        vr.reset()
        frames = []
        for img in vr:
            frames.append(vr.frame_number)
            if vr.frame_number >= max_frame:
                print(frames)
                break