Standard deviation patch core area (class level)

## Arguments

- landscape
A categorical raster object: SpatRaster; Raster* Layer, Stack, Brick; stars or a list of SpatRasters.

- directions
The number of directions in which patches should be connected: 4 (rook's case) or 8 (queen's case).

- consider_boundary
Logical if cells that only neighbour the landscape boundary should be considered as core

- edge_depth
Distance (in cells) a cell has the be away from the patch edge to be considered as core cell

## Details

$$CORE_{SD} = sd(CORE[patch_{ij}])$$ where \(CORE[patch_{ij}]\) is the core area in square meters of each patch.

CORE_SD is a 'Core area metric'. It equals the standard deviation of the core area of each patch belonging to class i. The core area is defined as all cells that have no neighbour with a different value than themselves (rook's case). The metric describes the differences among patches of the same class i in the landscape.

Because the metric is based on distances or areas please make sure your data
is valid using `check_landscape`

.

## References

McGarigal K., SA Cushman, and E Ene. 2023. FRAGSTATS v4: Spatial Pattern Analysis Program for Categorical Maps. Computer software program produced by the authors; available at the following web site: https://www.fragstats.org