A function to extend a spatialAtRisk object, used in interpolating the fft grid NOTE THIS DOES NOT RETURN A PROPER spatialAtRisk OBJECT SINCE THE NORMALISING CONSTANT IS PUT BACK IN.
extendspatialAtRisk(spatial)
a spatialAtRisk object inheriting class 'fromXYZ'
the spatialAtRisk object on a slightly larger grid, with zeros appearing outside the original extent.