[olg] add an exercise

[shlff]

Based on the reading group's work thanks @HengchengZhang , @orectique , @DiaPorntipa .

We could add an exercise to enhance audience on econ principles:

lecture-python-intro/lectures/olg.md

Line 679 in 94272e4

## Exercises

This could possibly be done by connecting the code to the model and implementations with different parameters.