Mean of core area (Core area metric)

## 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

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

CORE_MN is a 'Core area metric' and equals the mean of core areas of all patches 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).

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