title: Scroll UICollectionView by pixels or percent?
body: <p>Fairly simple question, but I'm having a surprisingly hard time finding an answer: is it possible to scroll a UICollectionView by anything other than the indexPath of a cell within the view?</p>
  
  <p>I've got this right now, in order to center a cell onscreen:</p>
  
  <pre><code>[_mainCollectionView scrollToItemAtIndexPath:indexPath atScrollPosition:UICollectionViewScrollPositionCenteredHorizontally animated:NO];
  [_mainCollectionView scrollToItemAtIndexPath:indexPath atScrollPosition:UICollectionViewScrollPositionCenteredVertically animated:NO];
  </code></pre>
  
  <p>...but I'd love to be able to tell it "scroll 40% to the right" or "scroll 100 pixels down", regardless of what cell is at that position.</p>
  
  <p>This seems like it MUST exist, but I just can't find it.  Can anyone help me out?</p>