title: array2d from bytestream without writing file to disk
body: <p>is it possible to initialise  <code>array2d</code> object in <code>dlib</code> from a byte stream? </p>
  
  <p>The thing is that I receive bytestream which contains png file from external source and I do not want to create temp file and then read image from this file into array. Is it possible to skip write-to-file part?</p>
  
  <p>Speaking the other way, is it possible to build array2d object from file kept in memory?</p>
  
  <p>Thanks</p>