Creates circular focal window around central pixel.

## Arguments

- radius
radius of circular window

- unit
unit for radius. Either "cell" (number of cells) or "map" for map units (e.g. meters).

- resolution
resolution of intended raster layer (one number or a vector of length 2). Only necessary if unit= "map"

- return_dismat
logical, if TRUE return a matrix of distances from focal cell instead of a matrix to pass to terra::focal.

## Value

a matrix of 1's and NA's showing which cells to include and exclude respectively in focal calculations, or if return_dismat=TRUE, a matrix indicating the distance from the focal cell. It also contains attributes attributes 'unit', 'scale', and 'shape' if return_dismat=FALSE, and if return_dismat=TRUE the attribute 'unit'.