title: Interpolate PANDAS df
body: <p>I know this subject was brought up a few times on stack overflow, however I'm still stumbling upon an interpolation problem.</p>
  
  <p>I have a complex dataframe of a set of columns, which could look something like this if simplified:</p>
  
  <pre><code>df_new = pd.DataFrame(np.random.randn(5,7), columns=[402.3, 407.2, 412.3, 415.8, 419.9, 423.5, 428.3])
  wl     = np.array([400.0, 408.2, 412.5, 417.2, 420.5, 423.3, 425.0])
  </code></pre>
  
  <p>So what I need to do is to interpolate column-wise, to the new assigned values of cols (wl), for each row.</p>
  
  <p>And how to get the new dataframe with columns ONLY containing values presented in the wl array?</p>