Return a square grid with values of the distance from the centre 
    of the grid to each gridpoint

def RadiusGrid(gridSize):
    """
    Return a square grid with values of the distance from the centre 
    of the grid to each gridpoint
    """
    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)