Mention in the documentation parameter discrepancies between book and code

[universmile]

It might be useful to enhance the documentation of some parameters (with the book as a reference), that would act as a sanity check and help ensure that some inconsistencies (e.g., Issue #60) are not caused by a parameter discrepancy between the code and the book.

For example :

• In Chapter 2, instead of 1900, 1970, and 2100, the authors use 0, 70, and 200, respectively. Thus, the unit of lt is years.
• By looking at the source code in Chapter 7 (cf. screenshot below), we see that ppgf2 is a constant (= 1). Instead, if we look at Chapter 6, ppgf2 is a switch. This had led to an instability perceived in Figure 7.20, which was fixed (thanks to @paulobruno) by setting that the change on variable ppgf2 in the book corresponds to ppgf21 in the code.

pol_parameters_7_20 = World3.Pollution.getparameters()
pol_parameters_7_20[:ppgf21] = 0.1