Copy edit of UKF chapter.

Also found a mistake in the text about computing the residual
of angles that was duplicated in the EKF chapter, so I fixed the
error in that chapter as well.

I pulled out the 'track in a circle' example as it is the usual
sort of textbook nonsense problem that no one cares about. I created
an 'old-content' Notebook in case anyone still wants to see it; I
may put more content there as I continue to edit.

code/ekf_internal.py

@@ -204,7 +204,7 @@ def show_radar_chart():
 
 
 
-    ax.annotate('$\Theta$ (', xy=(1.2, 1.1), color='b')
+    ax.annotate('$\Theta$', xy=(1.2, 1.1), color='b')
     ax.annotate('Aircraft', xy=(2.04,2.), color='b')
     ax.annotate('altitude', xy=(2.04,1.5), color='k')
     ax.annotate('X', xy=(1.5, .9))