Deploy st_transform_auto() function

[mpjashby]

This package already has an unexported function st_transform_auto() but this has not yet been deployed to make using other package functions easier. To make use of this function:

• Remove function and corresponding test file from .Rbuildignore.
• Export it.
• Add an option to kernel_density() (and functions that call it) to automatically transform lon/lat data to a UTM zone CRS to enable density estimation (and then transform back again).
• Integrate the function into the relevant chapters of Learn Crime Mapping with R.