Ch2: misc fixes

Here's chapter2. Again, please throw away any changes you don't like.

I revamped the tricky "density scatter plot", so it's easier to grasp and 
much faster to compute. not as pretty though IMO.

There's a trick you might not be aware of, if you append a semicolon
to the last line in a cell, the output will be supressed, but any plots will
be displayed, so you can get rid of that noise.

I had trouble with the density plot when using ipnb with a sympy profile,
not sure what that's all about.

in the section "Including observations in the Model" , you elided (somehwat) the difference between assigning a value and an array of values to a stochastic variable,
I would have appreciated a bit more contex at that point, although it becomes
clearer later.

Thanks again for all your work and making it freely available,
chapter3 will be along presently.

Yoval.