title: Group by 'Date' while calculating average on other column
body: <p>I have a dataframe with 3 columns: ID, Date, Data_Value reporting temperature records (Data_Value) day by day in a given period (Date - day by day) from different weather stations (ID). What I need is to 'group by' every single day and calculate the average temperature for each day, e.g.</p>
  
  <pre><code>ID      |   Date       | Data_Value
  ------------------------------------
  12345   |   02-05-2017 |  22
  12346   |   02-05-2017 |  24
  12347   |   02-05-2017 |  20
  12348   |   01-05-2017 |  18
  12349   |   01-05-2017 |  16
  </code></pre>
  
  <p>becomes:</p>
  
  <pre><code>ID      |   Date       | Data_Value
  ------------------------------------
  .....   |   02-05-2017 | 22
  .....   |   01-05-2017 | 17
  </code></pre>
  
  <p>Could someone help me out with this?</p>