Add hook for outline layer to PCB class

gerber/pcb.py

@@ -94,6 +94,12 @@ class PCB(object):
                               layer.layer_class in
                               ('top', 'bottom', 'internal')]))
 
+    @property
+    def outline_layer(self):
+        for layer in self.layers:
+            if layer.layer_class == 'outline':
+                return layer
+
     @property
     def layer_count(self):
         """ Number of *COPPER* layers