title: Subtracting after groupby
body: <p>I have a dataframe like this:</p>
  
  <pre><code>   Allotment    Date        NDII_Mean
     Arnstson     19900619    0.073023
     A_Annex      19900619    0.131290
     Arnstson     19900620    0.045553
     A_Annex      19900620    0.688850
  </code></pre>
  
  <p>and I want to group by <code>Allotment</code> and then subtract the <code>19900620</code> dates by the <code>19900619</code> dates.  </p>
  
  <p>I want my output to look something like this:</p>
  
  <pre><code>Allotment      NDII_Mean
  Arnstson       -0.02747
  A_Annex         0.55756 
  </code></pre>