Integer order radial-2D Gaussian function.

Calculates a rotated 2D gaussian (cylindrical)

y = gauss2D_R(X, Y, FWHM_x, FWHM_y, theta, order);

X = Matrix of x indices
Y = Matrix of y indices
FWHM_x = FWHM in x dimension
FWHM_y = FWHM in y dimension
theta = Rotation angle in degress (+ve = anticlockwise)
order = Gaussian order

Set:
X = ones(Ny, 1) * ((0:Nx-1)*dx - x0)
Y = ((0:Ny-1)*dy - y0) * ones(1, Nx);