Add material on `map_*()` to programming preliminaries

[jepusto]

It might be good to include an introduction to purrr:::map_*() functions, since we will be using them extensively throughout the book.

I have material to use.

[lmiratrix]

Possibly initial map since we use it for simple replication? but delay pmap? I don't want to have too much heavy programming before we get to the simulation structure, and I lean to "just in time" teaching of the programming. But perhaps that makes things too hard to find.

Another option is to have some of this stuff in appendices to make it more explicitly as reference material in support of the core book?

[jepusto]

I like the idea of just presenting the simple map() function and delaying pmap() until needed.

I agree just-in-time is a good principle to follow and it's good to defer heavier stuff until later when it can be better motivated.