aglc: computation of count_rate changed

Before:
       sum_i  C_i
 r = ----------------
      (sum_i DT_i)/n

 C_i is the number of counts on CCD i.
 DT_i is the exposure on CCD i.
 n is the number of CCDs.

Now:
             C_i
 r = sum_i ------
            DT_i