Fix bug with DSMC collisions in RZ (ECP-WarpX#5622)

I realized there is a bug in the DSMC module for RZ geometry where the
velocity vectors for the colliding pair was not rotated so that a proper
center-of-momentum calculation could be done. This PR fixes the bug.

To check that the fix in this PR works, I compared the azimuthal
velocity distribution for energetic ions created from NBI with finite
impact parameter (such that a net rotation should be imparted on the
ions), when simulated in 3d versus RZ:

3d result:

![image](https://github.com/user-attachments/assets/458f7e9c-b7b2-46ec-b456-8733ce959f94)

RZ result:

![image](https://github.com/user-attachments/assets/ce37a51c-f127-44be-b9b9-4f9a1d7d2cbb)

Source/Particles/Collision/BinaryCollision/DSMC/DSMCFunc.H

@@ -176,6 +176,7 @@ public:
                         m_process_count, m_scattering_processes_data, engine);
 
 #if (defined WARPX_DIM_RZ)
+                    /* Undo the earlier velocity rotation. */
                     amrex::ParticleReal const u1xbuf_new = u1x[I1[i1]];
                     u1x[I1[i1]] = u1xbuf_new*std::cos(-theta) - u1y[I1[i1]]*std::sin(-theta);
                     u1y[I1[i1]] = u1xbuf_new*std::sin(-theta) + u1y[I1[i1]]*std::cos(-theta);

Source/Particles/Collision/BinaryCollision/DSMC/SplitAndScatterFunc.H

@@ -154,6 +154,25 @@ public:
                 auto& uy2 = soa_products_data[1].m_rdata[PIdx::uy][product2_index];
                 auto& uz2 = soa_products_data[1].m_rdata[PIdx::uz][product2_index];
 
+#if (defined WARPX_DIM_RZ)
+                /* In RZ geometry, macroparticles can collide with other macroparticles
+                * in the same *cylindrical* cell. For this reason, collisions between macroparticles
+                * are actually not local in space. In this case, the underlying assumption is that
+                * particles within the same cylindrical cell represent a cylindrically-symmetry
+                * momentum distribution function. Therefore, here, we temporarily rotate the
+                * momentum of one of the macroparticles in agreement with this cylindrical symmetry.
+                * (This is technically only valid if we use only the m=0 azimuthal mode in the simulation;
+                * there is a corresponding assert statement at initialization.)
+                */
+                amrex::ParticleReal const theta = (
+                    soa_products_data[1].m_rdata[PIdx::theta][product2_index]
+                    - soa_products_data[0].m_rdata[PIdx::theta][product1_index]
+                );
+                amrex::ParticleReal const ux1buf = ux1;
+                ux1 = ux1buf*std::cos(theta) - uy1*std::sin(theta);
+                uy1 = ux1buf*std::sin(theta) + uy1*std::cos(theta);
+#endif
+
                 // for simplicity (for now) we assume non-relativistic particles
                 // and simply calculate the center-of-momentum velocity from the
                 // rest masses
@@ -213,6 +232,13 @@ public:
                 ux2 += uCOM_x;
                 uy2 += uCOM_y;
                 uz2 += uCOM_z;
+
+#if (defined WARPX_DIM_RZ)
+                /* Undo the earlier velocity rotation. */
+                amrex::ParticleReal const ux1buf_new = ux1;
+                ux1 = ux1buf_new*std::cos(-theta) - uy1*std::sin(-theta);
+                uy1 = ux1buf_new*std::sin(-theta) + uy1*std::cos(-theta);
+#endif
             }
         });