Improvements to exchangeable lecture

Some suggestions for improving https://python.quantecon.org/exchangeable.html

• There are SyntaxWarning errors that need to be fixed -- this is high priority
• The function learning_example is mainly plotting code and I suggest that switch to opening it with a toggle
• The graphs produced by that function are too small when stacked horizontally -- switch to vertical

Also, inside the function simulate, built within the function factory, the first two arguments are a and b. Instead, this should be a single argument that selects the distribution F or G.

In other words, a and b should be tied to F_a, F_b or G_a, G_b from the parameters in the function factory.

(The function factory sets up the two distribution. The simulate function simulates from one of these distributions. In calling it, one should not need to independently specify F and G again.)

For example, the first argument of simulate could be simulate(density='F', ...)

If the string 'F' is selected, the the simulation uses F_a and F_b.

Start with https://python.quantecon.org/exchangeable.html and review the SyntaxWarning messages, learning_example, the function factory, and its simulate function. Check the plotting layout and toggle behavior, then trace how simulate receives distribution parameters. Done means the warnings are fixed, plots use the requested layout and toggle, and simulate selects F or G without separately resupplying both distributions.