added callback example

examples/callback.py

@@ -0,0 +1,23 @@
+from niapy.callbacks import Callback
+from niapy.algorithms.basic import BatAlgorithm
+from niapy.task import Task
+# This is temporary fix to import module from parent folder
+# It will be removed when package is published on PyPI
+import sys
+
+sys.path.append('../')
+# End of fix
+
+
+class PrintMeanFitness(Callback):
+    def __init__(self):
+        super().__init__()
+
+    def after_iteration(self, population, fitness, best_x, best_fitness, **params):
+        print(fitness.mean())
+
+
+ba = BatAlgorithm(callbacks=[PrintMeanFitness()])
+griewank = Task('griewank', max_evals=1000)
+
+ba.run(griewank)