bugfix

From blog comment:
https://www.kawabangga.com/posts/1923#comment-16995

Author: laixintao

slic.py

@@ -11,7 +11,7 @@ def __init__(self, h, w, l=0, a=0, b=0):
         self.update(h, w, l, a, b)
         self.pixels = []
         self.no = self.cluster_index
-        self.cluster_index += 1
+        Cluster.cluster_index += 1
 
     def update(self, h, w, l, a, b):
         self.h = h
@@ -176,4 +176,4 @@ def iterate_10times(self):
     p = SLICProcessor('Lenna.png', 500, 5)
     p.iterate_10times()
     p = SLICProcessor('Lenna.png', 1000, 5)
-    p.iterate_10times()
+    p.iterate_10times()