Support structured bindings for VirtualDatum

[bernhardmgruber]

No description provided.

[bernhardmgruber]

This should allow us to write e.g. the nbody kernel as:

    template <typename VirtualParticleI, typename VirtualParticleJ>
    LLAMA_FN_HOST_ACC_INLINE void pPInteraction(VirtualParticleI&& pi, VirtualParticleJ pj)
    {
#ifdef USE_STRUCTURED_BINDING
        auto [piPos, piVel, _1] = pi;
        auto [pjPos, _2, pjMass] = pj;

        auto dist = piPos - pjPos;
        dist *= dist;
        auto [distX, distY, distZ] = dist;
        const FP distSqr = EPS2 + distX + distY + distZ;
        const FP distSixth = distSqr * distSqr * distSqr;
        const FP invDistCube = 1.0f / std::sqrt(distSixth);
        const FP sts = pjMass * invDistCube * TIMESTEP;
        piVel += dist * sts;
#else
        auto dist = pi(tag::Pos{}) - pj(tag::Pos{});
        dist *= dist;
        const FP distSqr = EPS2 + dist(tag::X{}) + dist(tag::Y{}) + dist(tag::Z{});
        const FP distSixth = distSqr * distSqr * distSqr;
        const FP invDistCube = 1.0f / std::sqrt(distSixth);
        const FP sts = pj(tag::Mass{}) * invDistCube * TIMESTEP;
        pi(tag::Vel{}) += dist * sts;
#endif
    }

This is great, because it allows us to navigate the VirtualDatum without tag types.

See also #141 for a version with asTuple.

Both versions produce identical assembly: https://godbolt.org/z/jEv33W