title: Get positions of max value(s) in a dataframe
body: <p>Given a dataframe like so:</p>
  <pre><code>df = pd.DataFrame({
      'col1': [1, 4, 7],
      'col2': [5, 2, 9],
      'col3': [3, 8, 6],
  })
  </code></pre>
  <p>How might I get the positions of the max n values?</p>
  <p>e.g.</p>
  <pre><code>max_val_positions(df, n=2)
  </code></pre>
  <p>Might give output:</p>
  <pre><code>[(col2, 2), (col3, 1)]  # [(column name, row index), ...]
  </code></pre>