title: Invalid value (printed as '--') in a numpy array from a netCDF file
body: <p>I read netCDF files through <code>netCDF4</code> into <code>python</code>. 
  I get arrays with invalid values that print as <code>--</code>, for example:</p>
  
  <pre><code>[[[-- -- -- ..., 0.0 0.0 0.0]
    [-- -- -- ..., 0.0 0.0 0.0]
    [-- -- -- ..., 0.0 0.0 0.0]]
  
   [[-- -- -- ..., 0.0 0.0 0.0]
    [-- -- -- ..., 0.0 0.0 0.0]
    [-- -- -- ..., 0.0 0.0 0.0]]]
  </code></pre>
  
  <p>Where do these values originate from, and how could I identify (and replace) them? Things such as <code>numpy.isnan</code> would return me <code>--</code> as result, too. I can use these arrays with <code>--</code> later without crash (I can for example plot them), but they seem to be problematic for some operations.</p>
  
  <p>For example, I used to do something like </p>
  
  <pre><code>numpy.mean(myarray, axis=(1,2)) # the tuple for `axis` is not very regular
  </code></pre>
  
  <p>on <code>myarray</code> originating from my netCDF files before without any problem, but I get <code>TypeError: tuple indices must be integers, not tuple</code> and a crash with these weird <code>--</code>-filled arrays...</p>
  
  <p>I am using <code>python 2.7.9</code> (and, in case relevant for the printing to screen format, <code>PyCharm</code> for code editing).</p>