Updates population according to crossover and fitness criteria.

def _update_pop(self, pop_size):
        """Updates population according to crossover and fitness criteria."""
        self.toolbox.generate()
        # simple bound checking
        for i in range(len(self.population)):
            for j in range(len(self.population[i])):
                if self.population[i][j] > 1:
                    self.population[i][j] = 1
                if self.population[i][j] < -1:
                    self.population[i][j] = -1
        self.assign_fitnesses(self.population)
        self.toolbox.update(self.population)
        self._model_count += len(self.population)
        return