Use amrex::getParticleCell More (#5557)

One leftover occurance in `ParticleReductionFunctor`.

Follow-up to #5118

Source/Diagnostics/ComputeDiagFunctors/ParticleReductionFunctor.cpp

@@ -69,6 +69,9 @@ ParticleReductionFunctor::operator() (amrex::MultiFab& mf_dst, const int dcomp,
                 get_particle_position(p, xw, yw, zw);
 
                 // Get position in AMReX convention to calculate corresponding index.
+                // Ideally this will be replaced with the AMReX NGP interpolator
+                // Always do x direction. No RZ case because it's not implemented, and code
+                // will have aborted
                 const auto [ii, jj, kk] = amrex::getParticleCell(p, plo, dxi).dim3();
 
                 // Fix dimensions since parser assumes u = gamma * v / c
@@ -97,20 +100,8 @@ ParticleReductionFunctor::operator() (amrex::MultiFab& mf_dst, const int dcomp,
                     // Ideally this will be replaced with the AMReX NGP interpolator
                     // Always do x direction. No RZ case because it's not implemented, and code
                     // will have aborted
-                    int ii = 0, jj = 0, kk = 0;
-                    const amrex::ParticleReal x = p.pos(0);
-                    const amrex::Real lx = (x - plo[0]) * dxi[0];
-                    ii = static_cast<int>(amrex::Math::floor(lx));
-#if defined(WARPX_DIM_XZ) || defined(WARPX_DIM_3D)
-                    const amrex::ParticleReal y = p.pos(1);
-                    const amrex::Real ly = (y - plo[1]) * dxi[1];
-                    jj = static_cast<int>(amrex::Math::floor(ly));
-#endif
-#if defined(WARPX_DIM_3D)
-                    const amrex::ParticleReal z = p.pos(2);
-                    const amrex::Real lz = (z - plo[2]) * dxi[2];
-                    kk = static_cast<int>(amrex::Math::floor(lz));
-#endif
+                    const auto [ii, jj, kk] = amrex::getParticleCell(p, plo, dxi).dim3();
+
                     // Fix dimensions since parser assumes u = gamma * v / c
                     const amrex::ParticleReal ux = p.rdata(PIdx::ux) / PhysConst::c;
                     const amrex::ParticleReal uy = p.rdata(PIdx::uy) / PhysConst::c;