This code did not work for Python 2.x becaus I was not
importing from future. While I was altering all the files
I updated the header to include license information.
I was saying 'blue curve' 'green curve', but those have no meaning,
especially if printed in black and white. Switched to explicit
$\sigma=0.2$.
Also added second std in the chart showing the 68-95-99.7 rule.
I made a separate filterpy.stats module becuase it made
little sense to import filterpy.common for stats
functions. This required a lot of changes in the notebooks
and supporting code.
