A function to generate an FFT grid and associated quantities including cell dimensions, size of extended grid, centroids,
FFTgrid(spatialdata, cellwidth, ext, boundingbox = NULL)
a list
a SpatialPixelsDataFrame object
width of computational cells
multiplying constant: the size of the extended grid: ext*M by ext*N
optional bounding box over which to construct computational grid, supplied as an object on which the function 'bbox' returns the bounding box