weighting issue: total number of population under 18 years old (nu18) not accurate

[bodiyang]

The total number of population under 18 years old is 94.116118(M) as calculated by calc0.weighted_total('nu18')

This number exceeds the actual number is 74.4(M), as can be found at this statistics of child number

Proposal: to include the total child number into the target in the weighting processes