Make chapter beginnings consistent

[Robinlovelace]

Currently some have this:

---
knit: "bookdown::preview_chapter"
---

They should all have prerequisites...

@Nowosad note my use of pacman's p_load() for package loading - you happy with that? I think it's the best solution and that the benefits justify the costs of having another (minor) dependency for the book.

[Nowosad]

Where can I find an example of your use of p_load?

[Robinlovelace]

Here - in the advanced plotting chapter:

https://github.com/Robinlovelace/rgeobook/blob/ae8fd5551512197695cf1da7001cbc8253450ea4/10-adv-map.Rmd#L21

Search term: https://github.com/Robinlovelace/rgeobook/search?utf8=%E2%9C%93&q=p_load&type=

[Nowosad]

I'm fine with that;)