title: Pandas: how to convert an index of int64 years to datetime
body: <p>I have an int64 index representing year values that I want to treat as a timestamp index:</p>
  
  <pre><code>df.index
  Int64Index([2001,2002,2003], dtype='int64')
  </code></pre>
  
  <p>How do I convert the index to a datetime timestamp in pandas?</p>