title: pandas add new row based on sum/difference of other rows
body: <p>df have</p>
  
  <pre><code>id    measure   t1  t2  t3
  1     savings   1    2   5
  1     income    10   15  14
  1     misc       5    5   5
  2     savings    3   6   12
  2     income     4   20  80
  2     misc       1   1    1
  </code></pre>
  
  <p>df want- add a new row to the measure for each id, called spend, calculated by subtracting measure=income - measure=savings, for each of the periods t1,t2,t3, for each id</p>
  
  <pre><code>id    measure   t1  t2  t3
  1     savings   1    2   5
  1     income    10   15  14
  1     misc      5     5   5
  1     spend     9    13  9
  2     savings    3   6   12
  2     income     4   20  80
  2     misc       1    1   1
  2     spend      1   14  68
  </code></pre>
  
  <p>Trying:</p>
  
  <pre><code>df.loc[df['Measure'] == 'spend'] =          
                          df.loc[df['Measure'] == 'income']-
                          (df.loc[df['Measure'] == 'savings'])
  </code></pre>
  
  <p>Failing because I am not incorporating groupby for desired outcome</p>