Enable stretching for the 110 case

[ia267]

Since the 110 case Dirchlet in y-direction, can we support stretching in y-direction? If it's something that'll be implemented later we could just add an error saying that it's currently not supported (and create an issue)

Originally posted by @ia267 in #266 (comment)

[ia267]

we can try adding this bit of code in 110 case in src/poisson_fft.f90

if (mesh%geo%stretched(2)) then
        self%stretched_y = .true.
        call self%stretching_matrix(mesh%geo, xdirps, ydirps, zdirps)
      end if