title: Pandas time series resampling with month and with group by column
body: <p>How to resample a dateindex with month and group by one column and aggregate mean of another column.</p>
  
  <p>Example of dataframe:</p>
  
  <pre><code>                bts_name    duration
  cleareddate     
  2019-01-19  1002_NUc_Marathalli 95
  2019-01-21  1002_NUc_Marathalli 188
  2019-02-11  1002_NUc_Marathalli 1332
  2019-04-12  1002_NUc_Marathalli 940
  2019-01-11  1003_IU2_Munnekolalu 73
  </code></pre>
  
  <p>I am trying to with frequency month and group by <code>bts_name</code> and <code>sum</code> the duration of each month.</p>