title: Find max value in a pandas dataframe that also has None's, Python 3.5
body: <p>I have a pandas dataframe setup like this:</p>
  
  <pre><code>     Group1      Group2      Group3
  0   0.04058678  0.04282689  0.06680679
  1   0.11657916  0.06695174  0.05153584
  2   0.08382576  0.03587087  0.08919266
  3   0.17477007  0.08141088  0.10727157
  4    0.0821453  0.08226264  0.06800853
  5   0.15685707        None  0.09467674
  6   0.08237982        None  0.14494069
  7         None        None  0.14541177
  8         None        None  0.12181681
  9         None        None  0.17966472
  10        None        None   0.1509818
  </code></pre>
  
  <p>I tried using df.max() to find the maximum value in the dataframe, but it doesn't work with this data and I think it is because of None in some fields.</p>
  
  <p>I get this error:</p>
  
  <pre><code>print(df.max())  
  TypeError: unorderable types: float() &gt; str()
  </code></pre>
  
  <p>How do I deal with None in this dataframe so that I can get the maximum value?</p>