title: Count and sort co-occurence matrix
body: <p>I have a co-occurrence matrix in pandas. How do I get the co-occurence values of all the combinations, sorted descending without looping?
  (I didn't write the values on the other side of the diagonal, but they are there, and hold mirrored values)</p>
  
  <p>Input:</p>
  
  <pre><code>  A  B  C  D  E  F
  A 0  1  0  1  2  0
  B    0  3  1  1  1
  C       0  1  8  9
  D          0  2  6
  E             0  9
  F                0
  </code></pre>
  
  <p>Output:</p>
  
  <pre><code>CF  9
  EF  9
  CE  8
  DF  6
  BC  3
  AE  2
  DE  2
  AB  1
  AD  1
  BD  1
  BE  1
  BF  1
  CD  1
  AC  0
  AF  0
  </code></pre>