added validation to check orphans dob within gestation period of fathers death

The longest and 2nd longest gestation periods are 375 days and 317 days respectively
http://www.newhealthguide.org/Longest-Pregnancy.html

Validation ensures gestation within 317 days.
This line can be omitted by amending L27 to `if gestation_offset && (gestation_offset < min_gestation_offset)`