title: No conversion from RGB to YUV
body: <p>I fail to find an easy-to-use function in any Python library (preferrably PIL) for conversion from RGB to YUV.
  Since I have to convert many images, I don't want to implement it myself (would be expensive without LUTs and so on).</p>
  
  <p>When I do the intuitive:</p>
  
  <pre><code>from PIL import Image
  img = Image.open('test.jpeg')
  img_yuv = img.convert('YUV')
  </code></pre>
  
  <p>I get an error:</p>
  
  <pre><code>ValueError: conversion from RGB to YUV not supported
  </code></pre>
  
  <p>Do you know why this is the case?
  Is there any efficieint implementation of that in python and maybe even PIL?</p>
  
  <p>I am no computer vision expert but I thought this ocnversion is standard in most of the libraries...</p>
  
  <p>Thanks,</p>
  
  <p>Roman</p>