*) Mod of Vanilla
*) Changed Population dynamics:
   1) added a connection from Population.y to births.u4
*) Changed Birth_Factors:
   1) Now receives 4 inputs instead of 3
   2) Added population as input u4 and output "population"
   3) Added "pseudo_ffw_var" as output
   4) defined "pseudo_ffw_var" as 0.5 * pop_15_44 / population (from an equation
      equating birth_rates in world3-v1 and birth_rates in world3-v3)
