title: Single column with value counts from multiple column dataframe
body: <p>I would like to sum the frequencies over multiple columns with pandas. The amount of columns can vary between 2-15 columns. Here is an example of just 3 columns:</p>
  
  <pre><code>code1    code2    code3
  27       5        56
  534      27       78
  27       312      55
  89       312      27
  </code></pre>
  
  <p>And I would like to have the following result:</p>
  
  <pre><code>code    frequency
  5       1
  27      4
  55      1
  56      2
  78      1
  312     2
  534     1
  </code></pre>
  
  <p>To count values inside one column is not the problem, just need a sum of all frequencies in a dataframe a value can appear, no matter the amount of columns.</p>