title: how to use AVCaptureSession to read a video from a file?
body: <p>I am writing an app that does some real-time video processing using an <code>AVCaptureSession</code> with a <code>AVCaptureVideoDataOutput</code> as output and an <code>AVCaptureDeviceInput</code> with the video file (it no longer needs to be in real-time) as input.</p>
  
  <p>Is it possible to use the video file as in input to the <code>AVCaptureSession</code> instead of the camera? 
  If it is not possible, what is the best method to process a video file using video capture of <code>opencv on iOS</code> (either simultaneously or sequentially)?</p>