Assigns initial fitnesses.

def initialize_pop(self):
        """Assigns initial fitnesses."""
        self.toolbox.register("individual", self.generate)
        self.toolbox.register("population", tools.initRepeat,
                              list, self.toolbox.individual)
        self.population = self.toolbox.population(n=self._params['popsize'])
        self.assign_fitnesses(self.population)
        self._params['model_count'] += len(self.population)