Reported average numbers can be wrong

[BoPeng]

If some replicates have ended, they may not show up in the individual list, and the final average can be the average for the rest of ther replicates.

0 should be recorded.

n_active_115.00	0, 12, 0, 0, 5, 1
n_active_116.00	0, 12, 0, 5, 1
n_active_117.00	0, 12, 4, 1

[BoPeng]

NOTE: All `avg_` fields have average over existing replicates, and all replicates. For
example, if at time 100, 8 of the replicate simulations have completed and the rest 2
have population size 5, and 10, the average population size will be reported as
`7.5` (`5+10/2`) and `0.15` (`5+10/10`).