Implement spectral Hessian

We would like to implement a spectral Hessian computation in the FFTPoissonSolver class, for Open BCs. This follows the same method as the spectral gradient in FFTPoissonSolver and FFTPeriodicPoissonSolver, where we take advantage of being in Fourier space where the derivative is just a multiplication with a wavevector. This is beneficial to the Langevin project, since we would get better accuracy than with the finite differences Hessian and we avoid the need for a one-sided Hessian to deal with the boundary conditions.