title: DataFrame: add column whose values are the quantile number/rank of an existing column?
body: <p>I have a DataFrame with some columns. I'd like to add a new column where each row value is the quantile rank of one existing column. </p>
  
  <p>I can use DataFrame.rank to rank a column, but then I don't know how to get the quantile number of this ranked value and to add this quantile number as a new colunm.</p>
  
  <p>Example: if this is my DataFrame </p>
  
  <pre><code>df = pd.DataFrame(np.array([[1, 1], [2, 10], [3, 100], [4, 100]]), columns=['a', 'b'])
  
     a    b
  0  1    1
  1  2   10
  2  3  100
  3  4  100
  </code></pre>
  
  <p>and I'd like to know the quantile number (using 2 quantiles) of column b. I'd expect this result:</p>
  
  <pre><code>   a    b  quantile
  0  1    1    1
  1  2   10    1
  2  3  100    2
  3  4  100    2
  </code></pre>