Add in chapter/appendix on alternative code/class structuring

[Bergam0t]

Might be good to have a chapter on other common ways you can structure models.

Particularly thinking about referencing the way used by Tom Monks - this opens up a wide range of additional sample models to people, but giving them a hand with how to interpret a simple example and comparing/contrasting with the same model written using the HSMA structure could be valuable

[amyheather]

I think this sounds a fab idea!