#2318 Patch for hann dgrid3d kernel

[Eisuke Kawashima]

Dear all,

At present hann kernel function for dgrid3d is implemented as
w(d) = 0.5 * (1 − cos(2*π*d)) * 1(d<1),
which seems to wrong as it takes 0 at d=0.
The function should be
w(d) = 0.5 * (1 + cos(π*d)) * 1(d<1).
Please find the attached patch.

Best

[Ethan Merritt]

Second error in the dgrid3d code reported recently, 12 years after the code was added. See Bug #2277. It must not have been tested much initially and not used much since then.
Thanks.

[Ethan Merritt]

Applied for 5.4 and 5.5