Uses the population description and detectability details to generate an 
instance of the population. Note that if the first argument supplied is 
of class Population.Description rather than class Simulation then a second 
argument detectabity must also be supplied and must be an object of 
class Detectability.
generate.population(object, ...)# S4 method for Population.Description
generate.population(object,
  detectability, region.obj = NULL)
# S4 method for Simulation
generate.population(object, ...)
an object of class Simulation or Population.Description
when this is called on an object of class Population.Description the additional arguments detectability and region.obj should also be supplied
object of class Detectability (optional - only required if object is of class Population.Description)
the region object for the population (optional - only required if object is of class Population.Description)
an object of class Population