title: Add/Modify index of Pandas DataFrame
body: <p>I have a multi-index Pandas DataFrame <code>data</code> which has indexes <code>ID</code> and <code>Frequency</code> (Hz), and column name <code>Angles</code>. For each <code>ID</code> data set , the <code>Frequency</code> index ranges from 4 to 1000 in steps of 4.</p>
  
  <pre><code>Angles                            1          2          3          4  \
  ID             Frequency                                               
  11111          4.0              2.1        5.4        3.1        6.4   
                 8.0              1.1        7.9        4.5        6.3   
                 12.0             5.2        5.1        9.6       11.2
                 16.0             6.7        6.1        2.9        7.3   
                 20.0             1.1        6.1        4.7        6.9
  ...                             ...        ...        ...        ...  
  11120          992.0            9.2        5.4        2.7        9.2
                 996.0            8.1        8.9        3.1        4.5
                 1000.0           2.1        1.8        5.5        8.4
  </code></pre>
  
  <p>I want to add an additional <code>Rad/s</code> index column which multiples each value in the <code>Frequency</code> index by 2π, giving something like this:</p>
  
  <pre><code>Angles                                 1          2          3          4  \
  ID          Frequency    Rad/s                                              
  11111       4.0          25.1        2.1        5.4        3.1        6.4   
              8.0          50.3        1.1        7.9        4.5        6.3   
              12.0         75.4        5.2        5.1        9.6       11.2
              16.0         100.5       6.7        6.1        2.9        7.3   
              20.0         125.7       1.1        6.1        4.7        6.9
  ...                                  ...        ...        ...        ...  
  11120       992.0        6232.9      9.2        5.4        2.7        9.2
              996.0        6258.1      8.1        8.9        3.1        4.5
              1000.0       6283.2      2.1        1.8        5.5        8.4
  </code></pre>
  
  <p>I've looked into changing the <code>Frequency</code> index into a regular column and applying the multiplication after but I was wondering if there is a better way of doing this?</p>