title: pandas get percentile of value withing
body: <p>I have a dataframe:</p>
  <pre><code>d = [f1  f2  f3 
       1    2   3 
       5    1   2 
       3    3   1 
       2    4   7
       ..  ..  ..]
  </code></pre>
  <p>I want to add, per feature, the percentile of the value for this feature in the row (for subset of features).
  So for <code>subset = [f1,f2,f3]</code> my dataframe will be</p>
  <pre><code>new_d =[f1   f2   f3  f1_per   f2_per   f3_per
           1    2   3    0         0.25     0.5
           5    1   2    1          0       0.25
           3    3   1    0.5        0.5     0
           2    4   5    0.25      0.75     1
           4    5   4    0.75       1       0.75]
  </code></pre>
  <p>What is the best way to do so?</p>