Use IntVectExpand or dim3 to reduce dependency on AMREX_SPACEDIM

[EZoni]

Another way to write

    const int rj = rr[0];
    const int rk = (AMREX_SPACEDIM > 1) ? rr[1] : 1;
    const int rl = (AMREX_SPACEDIM > 2) ? rr[2] : 1;

would be

    const amrex::IntVectND<3> rr_ex = amrex::IntVectExpand<3>(rr, 1);
    const auto [rj, rk, rl] = rr_ex;

or 

    const auto [rj, rk, rl] = amrex::IntVectExpand<3>(rr, 1);

or 

    const auto [rj, rk, rl] = rr.dim3(1);

(untested)

Originally posted by @AlexanderSinn in #5246 (comment)

[EZoni]

Cc: @lucafedeli88 @dpgrote

[lucafedeli88]

On a second thought, this one is actually quite transparent:

    const auto [rj, rk, rl] = amrex::IntVectExpand<3>(rr, 1);