Add a lecture on wealth dynamics with ideosyncratic shocks

[jstac]

Take this lecture and focus on simulation https://python.quantecon.org/wealth_dynamics.html

This lecture should go after #14 , so there is no need to explain what the Lorenz curve and Gini coefficient are.

However, we need to carefully explain that the wealth distribution "becomes stationary" --- just by simulation, not maths

Then we explain that we are looking at inequality at the stationary distribution.

Investigate how it varies with parameters.

Simplify discussion in the last exercise --- remove mention of Kesten Goldie theorem and just ask for a rank-size plot.