Hotfix fft 1D 1D

Description

Fix the kernel for 1D periodic FFTs. The G=0 component was not calculated and was using instead G=1. All the other kernel seem the do the pading of frequencies properly.

News snippet

Fix the kernel for 1D periodic FFTs.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

src/poisson/poisson_fft.F90

@@ -771,7 +771,7 @@ contains
     type(cube_t),        intent(inout) :: cube
     FLOAT,               intent(in)    :: poisson_soft_coulomb_param
 
-    integer            :: ix
+    integer            :: ix, ixx
     FLOAT              :: g
     FLOAT, allocatable :: fft_coulb_fs(:, :, :)
 
@@ -783,7 +783,7 @@ contains
     ! Fourier transform of Soft Coulomb interaction.
     do ix = 1, cube%fs_n_global(1)
       ixx = pad_feq(ix, cube%rs_n_global(1), .true.)
-      g = (ixx+this%qq(1))*M_PI/mesh%sb%lsize(1)
+      g = (ixx + this%qq(1))*M_PI/mesh%sb%lsize(1)
       if(abs(g) > CNST(1e-6)) then
         fft_coulb_fs(ix, 1, 1) = M_TWO * loct_bessel_k0(poisson_soft_coulomb_param*abs(g))
       end if