code: def RadiusGrid(gridSize):
      
      x,y=np.mgrid[0:gridSize,0:gridSize]
      x = x-(gridSize-1.0)/2.0
      y = y-(gridSize-1.0)/2.0
      return np.abs(x+1j*y)
docs: Return a square grid with values of the distance from the centre 
      of the grid to each gridpoint