title: ase.visualize.view misses pygtk
body: <p>On Ubuntu 16.04 machinge w/ XFCE desktop i have installed <code>python3</code> and <code>pip</code> with the command <code>sudo apt install python3-pip</code>. I then installed <code>numpy</code> and <code>ase</code> (atomic Simulation Environment) using <code>sudo -H python3 -m pip install --upgrade numpy ase</code>. No apparent problems. However, running this <code>mwe.m</code>:</p>
  
  <pre><code>from ase import Atoms
  from ase.build import fcc111
  
  slab = fcc111('Cu', size=(4, 4, 2), vacuum=10.0)
  
  from ase.visualize import view
  view(slab)
  </code></pre>
  
  <p>results in the following: </p>
  
  <pre><code>$ python3 mwe.m
  $ ImportError: No module named 'pygtk'
  To get a full traceback, use: ase-gui --verbose
  </code></pre>
  
  <p>The problem is in the <code>view</code> command that depends on <code>ase-gui</code> which seem to depend on <code>pygtk</code>.
  My goal is to go through <a href="https://wiki.fysik.dtu.dk/ase/tutorials/surface.html" rel="nofollow">this tutorial</a>. Of course I'm a novice, any help is appreciated. How can i overcome this problem?</p>