title: Load RGB image in python from ESA Sentinel-2 product and save with openCV
body: <p>As from ESA snap, for a RGB image we should put Band 4 into Red Channel, Band 3 into Green Channel and Band 2 into Blue Channel. How can we read those bands with python into a <code>numpy</code> array so we could do whatever image processing we want and then save a RGB image on disk?</p>
  
  <pre><code>from snappy import Product
  from snappy import ProductIO
  import numpy as np
  import cv2
  
  product = ProductIO.readProduct(path_to_product)
  
  width = product.getSceneRasterWidth()
  height = product.getSceneRasterHeight()
  
  # Natural colors 
  red = product.getBand('B4')
  green = product.getBand('B3')
  blue = product.getBand('B2')
  </code></pre>
  
  <p>For example here is the type of one of the above variables (same for the others):</p>
  
  <pre><code>type(red)
  # org.esa.snap.core.datamodel.Band
  </code></pre>
  
  <p>How can I get numpy arrays from these data, and subsequently save them to disk as jpg images?</p>