title: Points not showing in simple pptk example
body: <p>I have just started using pptk the open source point cloud viewer.
  It worked perfectly when I used the example of displaying the 
  bildstein_station1_xyz_intensity_rgb.txt as per the tutorial so I do not think there can be much wrong but when I try to display 100 random points the viewer opens and says it is displaying the points but I can see nothing.
  The code is that given in the tutorial.</p>
  
  <pre><code>import pptk
  import numpy as np
  
  xyz = pptk.rand(10, 3)
  
  v = pptk.viewer(xyz)
  v.set(point_size=0.5)
  </code></pre>
  
  <p>I have tried changing the point_size parameter without success.
  Has anybody used this library and have any idea what I am doing wrong</p>