Documentation for SquaredPhi Hamiltonian shows an incorrect expression

[mortele]

The derived expression for the SquaredPhi Hamiltonian in terms of the deviations dϕ=ρ-ϕ from the average density ρ in the documentation is incorrect, should be

    .. math::

        w[\\tilde\\phi] = \\frac{1}{2\\kappa\\rho_0}
            \\left(
                -2N_k\\rho_0\\left[
                    \\sum_k \\mathrm{d}\\tilde\\phi_k
                \\right]
                +
                N_k^2\\rho_0^2
                +
                \\sum_k \\mathrm{d}\\tilde\\phi_k^2
                +
                2\\sum_{k\\not=l}
                    \\mathrm{d}\\tilde\\phi_k \\mathrm{d}\\tilde\\phi_l
            \\right),