title: Setting freq of pandas DatetimeIndex after DataFrame creation
body: <p>Im using pandas datareader to get stock data. </p>
  
  <pre><code>import pandas as pd
  import pandas_datareader.data as web
  ABB = web.DataReader(name='ABB.ST', 
                       data_source='yahoo',
                       start='2000-1-1')
  </code></pre>
  
  <p>However by default freq is not set on the resulting dataframe.
  I need freq to be able to navigate using the index like this:</p>
  
  <pre><code>for index, row in ABB.iterrows():
      ABB.loc[[index + 1]]
  </code></pre>
  
  <p>If freq is not set on DatetimeIndex im not able to use <code>+1</code> etc to navigate.</p>
  
  <p>What I have found are two functions <code>astype</code> and <code>resample</code>. Since I already know to freq <code>resample</code> looks like overkill, I just want to set freq to daily.</p>
  
  <p>Now my question is how can i use astype on ABB to set freq to daily?</p>