Core: ShapeModel AABB is not appropriate for fixed ellipsoid integration region type

peak_end scaled by sigma is only applicatble to variable ellipsoids. What is the equivalent for fixed_peak_end?

    if (_params->kabsch_coords) {
        const Eigen::Vector3d sigma(_params->sigma_d, _params->sigma_d, _params->sigma_m);
        aabb.setLower(-_params->peak_end * sigma);
        aabb.setUpper(_params->peak_end * sigma);
    } else {
        const Eigen::Vector3d dx(_params->nbins_x, _params->nbins_y, _params->nbins_z);
        aabb.setLower(-0.5 * dx);
        aabb.setUpper(0.5 * dx);
    }