Generate a box surrounding a point object

`sbox(pts, xfrac = .1, yfrac = .1)`

## Arguments

- pts
A points data set

- xfrac
The fraction of the width of the point pattern by which the box will
surround the point pattern to the left and right.

- yfrac
The fraction of the height of the point pattern by which the box will
surround the point pattern to the top and bottom.

## Value

A points data set of four points giving the coordinates of the surrounding box

## References

Rowlingson, B. and Diggle, P. 1993 Splancs: spatial point pattern analysis
code in S-Plus. Computers and Geosciences, 19, 627-655;
the original sources can be accessed at:
https://www.maths.lancs.ac.uk/~rowlings/Splancs/. See also Bivand, R. and
Gebhardt, A. 2000 Implementing functions for spatial statistical analysis
using the R language. Journal of Geographical Systems, 2, 307-317.