add assertion to main block in vector.py as an example of simple test

src/fancymath/vector.py

@@ -32,7 +32,10 @@ class Vector(object):
 
 
 if __name__ == "__main__":
-    v1 = Vector(2.0, 13.0, -1.0)
+    v1 = Vector(3, 4, 0)
     print(v1, " magnitude is", v1.abs())
     v2 = Vector(1.0, 2.0, 3.0)
     print("v1.dot(v2) =", v1.dot(v2))
+
+    assert v1.abs() == 5
+    print("Tests passed!")