Adapt book sections into vignettes

[mpjashby]

Adapt content from Learn Crime Mapping with R into vignettes that would be suitable for users of the package.

• Choosing suitable parameters for hotspot maps (bandwidth, cell size, etc.). For KDE see Chapter 6 and for Gi* see Chapter 11.
• Troubleshooting KDE surfaces (e.g. insufficient points producing unhelpful hotspots, bandwidth too big/small). For bandwidth and cell size see Section 6.2.

The vignettes should de-emphasise any content that is specific to crime mapping, since that is the focus of the book but might not be the focus of readers of the vignettes.