Extensive addtions to UKF chapter.

I think I finally arrived at a good ordering of material.
Started with implementing a linear problem just so we can
see how it differs from the linear KF, then added problems
step by step. Got rid of most of the poor performing filters.

.gitignore

@@ -1,5 +1,7 @@
 # test document not for distribution
 short.pdf
+10_Unscented_Kalman_Filter_.ipynb
+tmp.ipynb
 
 # Byte-compiled / optimized / DLL files
 __pycache__/