title: making numpy.nanargmin return nan if column is all nan
body: <p>Is it possible to use <code>numpy.nanargmin</code>, so that it returns <code>numpy.nan</code>, on columns where there are only nans in them. Right now, it raises a <code>ValueError</code>, when that happens. And i cant use <code>numpy.argmin</code>, since that will fail when there are only a few nans in the column.</p>
  
  <p><a href="http://docs.scipy.org/doc/numpy/reference/generated/numpy.nanargmin.html" rel="nofollow">http://docs.scipy.org/doc/numpy/reference/generated/numpy.nanargmin.html</a> says that the <code>ValueError</code> is raised for all-nan slices. In that case, i want it to return numpy.nan (just to further mask the "non-data" with nans)</p>
  
  <p>this next bit does this, but is super-slow and not really pythonic:</p>
  
  <pre><code>for i in range(R.shape[0]):
      bestindex = numpy.nanargmin(R[i,:])
      if(numpy.isnan(bestindex)):
          bestepsilons[i]=numpy.nan
      else:
          bestepsilons[i]=epsilon[bestindex]
  </code></pre>
  
  <p>This next bit works too, but only if no all-nan columns are involved:</p>
  
  <pre><code>ar = numpy.nanargmin(R, axis=1)
  bestepsilons = epsilon[ar]
  </code></pre>
  
  <p>So ideally i would want this last bit to work with all-nan columns as well</p>