title: Get streak within groupby Pandas
body: <p>I'm looking for a method to calculate the streaks of groups.
  For each group, if the previous 'Result' is the same as the current 'Result' then those two results get added together. If the previous 'Result' is different to the current 'Result' or if there is no previous result, then it should return the current result.</p>
  <p>Any help is appreciated</p>
  <pre><code>df_input = pd.DataFrame()
  df_input['Name'] = ['A','B','A','A','C','C','B','A','B','C']
  df_input['Result'] = [1,1,-1,-1,1,1,-1,1,-1,1]
  
  df_output = pd.DataFrame()
  df_output['Name'] = ['A','B','A','A','C','C','B','A','B','C']
  df_output['Result'] = [1,1,-1,-1,1,1,-1,1,-1,1]
  df_output['Streak'] = [1,1,-1,-2,1,2,-1,1,-2,3]
               
  </code></pre>