Moved all code to \code subdirectory.

All these changes are to make the book easier to run from
cloud.sagemath.org. You can share individual notebooks there,
but not entire projects.By putting everything under code at the
user only needs to grab that one directory.

More on sagemath later, if I decide to pursue that as a delivery
mechanism...

code/nonlinear_internal.py

@@ -12,7 +12,7 @@ import numpy as np
 
 
 def plot1():
-
+    P = np.array([[6, 2.5], [2.5, .6]])
     stats.plot_covariance_ellipse((10, 2), P, facecolor='g', alpha=0.2)