Chapter reordering; nonlinear chapter added.

I wrote a chapter introducing the problem of nonlinearity in KF because this
information is common to the EKF, UKF, etc. Then I reordered chapters. I want
people ot learn the UKF before the EKF. While I was doing this, I moved the
H Infinity and Ensemble filters to later chapters because they are mostly
curiosities in this book.

experiements/zarchan_ball.ipynb

@@ -0,0 +1,50 @@
+{
+ "metadata": {
+  "name": "",
+  "signature": "sha256:cfd803b3edb531968a1e9983b9d281824050b61d08da023ead5045daf7c8c4e7"
+ },
+ "nbformat": 3,
+ "nbformat_minor": 0,
+ "worksheets": [
+  {
+   "cells": [
+    {
+     "cell_type": "code",
+     "collapsed": false,
+     "input": [
+      "import sympy\n",
+      "from sympy import Symbol\n",
+      "\n",
+      "from sympy.abc import x, g, b\n",
+      "\n",
+      "xdd = (0.0034*g*sympy.exp(-x/22000)*sympy.diff(x)**2) / (2*b) - g\n",
+      "y=sympy.symbols('y',cls=sympy.Function)\n",
+      "\n",
+      "xdd = sympy.diff(y(x), x, x) - (0.0034*g*sympy.exp(-x/22000)*sympy.diff(x)**2)  + g\n",
+      "\n",
+      "\n",
+      "sympy.pprint(xdd)\n"
+     ],
+     "language": "python",
+     "metadata": {},
+     "outputs": [
+      {
+       "output_type": "stream",
+       "stream": "stdout",
+       "text": [
+        "               -x              \n",
+        "              \u2500\u2500\u2500\u2500\u2500     2      \n",
+        "              22000    d       \n",
+        "g - 0.0034\u22c5g\u22c5\u212f      + \u2500\u2500\u2500(y(x))\n",
+        "                        2      \n",
+        "                      dx       \n"
+       ]
+      }
+     ],
+     "prompt_number": 10
+    }
+   ],
+   "metadata": {}
+  }
+ ]
+}