make the Gentle introduction even gentler

[sbenthall]

There are several things that feel a little arbitrary in the Gentle Introduction notebook:

• the use of single element lists for 'time varying' parameters in an infinite horizon model
• the need to run updateIncomeProcess() after changing an income parameter

These seem like issues that could be smoothed over with a little more work on the design.

[mnwhite]

Drawing attention to the fact that some model inputs are "raw" and some are "constructed" is an important point, and I don't think we want to avoid it in the introductory notebook(s).