title: interpolation on pandas dataframe columns
body: <p>I need to do <strong>interpolation</strong> between 2 columns of <code>pandas.DataFrame</code>, to fill the column between them. 
  Here are a few rows of my <code>data frame</code>, the column to be filled is <code>col2</code>:</p>
  
  <pre><code>col1  col2  col3
  2.35    1   2.37
  2.47    1   2.49
  2.51    1   2.53
  2.57    1   2.58
  2.54    1   2.57
  </code></pre>
  
  <p>So for interpolation I want to use <code>numpy.interp(x,xp,fp)</code>, but I can't figure out how to organize my data so that I will be able to use it. That is because the interpolation should be between <code>col1</code> and <code>col3</code> <strong>for each row.</strong>
  For example, for the first row I need it to look like that: </p>
  
  <pre><code>xp=[1,3]
  fp=[2.47,2.49]
  x=2
  y=numpy.interp(x,xp,fp)
  </code></pre>
  
  <p>and then fill first row of <code>col2</code> with <code>y</code>. And I need to do that again and again for each row.
  How ?</p>