title: Sum a range of cells in a single column in pandas dataframe
body: <p>I have three columns in a DataFrame.  I want to take the number in the Streak_Count column and sum up that number of cells from the returns in the MON TOTAL.  The result is displayed in the WANTED RESULT as shown below.  The issue I cant figure out is summing the number of cells which can be any number>> in this example between 1 and 4.  </p>
  
  <pre><code>              MON TOTAL STREAK_COUNT    WANTED RESULT
  1/2/1992       1.123077       1          1.123077 (only 1 so 1.12)
  2/3/1992      -1.296718       0 
  3/2/1992      -6.355612       2          -7.65233 (sum of -1.29 and -6.35)
  4/1/1992       5.634692       0 
  5/1/1992       4.180605       2          9.815297 (sum of 5.63 and 4.18)
  7/1/1992      -0.101016       0 
  8/3/1992      -0.706125       2         -0.807141 (sum of -.10 and -.706)
  10/1/1992      0.368579       0 
  11/2/1992      3.822277       0 
  1/4/1993       2.233359       0 
  2/1/1993       15.219644      4         21.643859
  3/1/1993       -2.647693      1         -2.647693
  4/1/1993       1.599094       1         1.599094
  </code></pre>