Implement grid zoning

[dabreegster]

Not sure if the R zb has this yet, but it seems like another obvious scheme people use. This one has much easier math, of course. If the number of parameters isn't as overwhelming -- maybe it's just center, the square grid size, and an optional boundary -- then probably a separate Options struct with Default is overkill.

[Robinlovelace]

It does. Reproducible example:

# install.packages("zonebuilder")
library(zonebuilder)
grid = zb_quadrat(x = london_a(), ncol = 5, nrow = 5)
plot(grid)

^{Created on 2021-03-20 by the reprex package (v1.0.0)}

Heads-up @mtennekes I'll add this to the README.