title: Pandas DataFrame merge choosing the higher value
body: <p>I have two DataFrames like this: </p>
  
  <pre><code>         1          2          3 
  0   61.579   0.000000  47.279861
  1    0.000   0.000000   0.000000
  2   62.700   9.180000  48.479861
  3   56.100  40.180000  71.679861
  4   73.100  50.930000  71.679861
  5   88.300  37.930000  36.479861 
  </code></pre>
  
  <p>I need to merge them choosing each time the higher value. All the values are float.
  Any ideas? I have to loop on the DataFrames?</p>