Update position in 2nd pass for tracer advect with Ucc. (#3496)

## Summary

In the Ucc advection routine for tracer particles, the 2nd advection
pass did not increment the particle position. This change fixes the
issue, and it is now consistent with the leapfrog approach used in the
Umac advection routine.

## Additional background

## Checklist

The proposed changes:
- [X] fix a bug or incorrect behavior in AMReX
- [ ] add new capabilities to AMReX
- [ ] changes answers in the test suite to more than roundoff level
- [ ] are likely to significantly affect the results of downstream AMReX
users
- [ ] include documentation in the code and/or rst files, if appropriate

Src/Particle/AMReX_TracerParticles.cpp

@@ -168,7 +168,7 @@ TracerParticleContainer::AdvectWithUcc (const MultiFab& Ucc, int lev, Real dt)
                 {
                     for (int dim=0; dim < AMREX_SPACEDIM; dim++)
                     {
-                        p.rdata(dim) = p.rdata(dim) + static_cast<ParticleReal>(dt*v[dim]);
+                        p.pos(dim) = p.rdata(dim) + static_cast<ParticleReal>(dt*v[dim]);
                         p.rdata(dim) = v[dim];
                     }
                 }