Calculate and return total fitness separately

src/population.py

@@ -9,8 +9,19 @@ class Population(object):
             self.members.append(Individual())
 
     def fitness_function(self):
+        for member in self.members:
+            member.fitness_function()
+
+    def total_fitness(self):
         total = 0
         for member in self.members:
-            total += member.fitness_function()
+            total += member.fitness
         return total
 
+    def roulette(self):
+        total = self.total_fitness()
+        for member in self.members:
+            total -= member.fitness
+            if total <= 0:
+                pass
+                # todo: add to crossover list