Fixed path issues, improved gaussian discussion.

Chapter was not adding ../code to the search path. Multivarite discusion about the ellipses
was a bit unclear about the relationship of the standard deviation.

code/DogSensor.py

@@ -0,0 +1,26 @@
+# -*- coding: utf-8 -*-
+"""
+Created on Sun May 11 13:21:39 2014
+
+@author: rlabbe
+"""
+
+from __future__ import print_function, division
+
+import numpy.random as random
+import math
+
+class DogSensor(object):
+
+    def __init__(self, x0=0, velocity=1, noise=0.0):
+        """ x0 - initial position
+            velocity - (+=right, -=left)
+            noise - scaling factor for noise, 0== no noise
+        """
+        self.x = x0
+        self.velocity = velocity
+        self.noise = math.sqrt(noise)
+
+    def sense(self):
+        self.x = self.x + self.velocity
+        return self.x + random.randn() * self.noise