title: Check if all elements in a group are equal using pandas GroupBy
body: <p>Is there a pythonic way to group by a field and check if all elements of each resulting group have the same value?</p>
  
  <p>Sample data:</p>
  
  <pre><code>              datetime rating  signal
  0  2018-12-27 11:33:00     IG       0
  1  2018-12-27 11:33:00     HY      -1
  2  2018-12-27 11:49:00     IG       0
  3  2018-12-27 11:49:00     HY      -1
  4  2018-12-27 12:00:00     IG       0
  5  2018-12-27 12:00:00     HY      -1
  6  2018-12-27 12:49:00     IG       0
  7  2018-12-27 12:49:00     HY      -1
  8  2018-12-27 14:56:00     IG       0
  9  2018-12-27 14:56:00     HY      -1
  10 2018-12-27 15:12:00     IG       0
  11 2018-12-27 15:12:00     HY      -1
  12 2018-12-20 15:14:00     IG       0
  13 2018-12-20 15:14:00     HY      -1
  14 2018-12-20 15:50:00     IG      -1
  15 2018-12-20 15:50:00     HY      -1
  16 2018-12-27 13:26:00     IG       0
  17 2018-12-27 13:26:00     HY      -1
  18 2018-12-27 13:44:00     IG       0
  19 2018-12-27 13:44:00     HY      -1
  20 2018-12-27 15:06:00     IG       0
  21 2018-12-27 15:06:00     HY      -1
  22 2018-12-20 15:48:00     IG       0
  23 2018-12-20 15:48:00     HY      -1
  </code></pre>
  
  <p>The grouping part can be done by</p>
  
  <pre><code>df.groupby([datetime.dt.date,'rating'])
  </code></pre>
  
  <p>However, I'm sure there must be a simple way to leverage the grouper and use a transform statement to return 1 if all the values from <code>signal</code> are the same. </p>
  
  <p>Desired output</p>
  
  <pre><code>2018-12-20  HY            True
              IG            False
  2018-12-27  HY            True
              IG            True
  </code></pre>