cellsFromExtent: Cells from extent, and vice versa
Description
cellsFromExtent returns the cell numbers for a Raster* object that are within a specfied extent (rectangular area), supply an object of class Extent, or another Raster* object.
extentFromCells returns an Extent object from a Raster* object and cell numbers. All cells are within the returned Extent.
An object of class Extent (which you can create with newExtent(), or another Raster* object )
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Logical. If TRUE, NA is returned for (virtual) cells implied by bndbox,
that are outside the RasterLayer (object). If FALSE, only cell numbers for the area where object and bndbox
overlap are returned (see intersect)