ch01-12: clean up by @eumiro

01-data-model/vector2d.doctest

@@ -0,0 +1,12 @@
+>>> from vector2d import Vector
+>>> v1 = Vector(2, 4)
+>>> v2 = Vector(2, 1)
+>>> v1 + v2
+Vector(4, 5)
+>>> v = Vector(3, 4)
+>>> abs(v)
+5.0
+>>> v * 3
+Vector(9, 12)
+>>> abs(v * 3)
+15.0