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.
Usage
extendspatialAtRisk(spatial)
Arguments
spatial
a spatialAtRisk object inheriting class
'fromXYZ'
Value
the spatialAtRisk object on a slightly larger grid, with
zeros appearing outside the original extent.