Expose the index of the neighboring particle to users of the iterator. (

AMReX-Codes#1628)

The proposed changes:
- [ ] fix a bug or incorrect behavior in AMReX
- [x] 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
- [ ] are described in the proposed changes to the AMReX documentation, if appropriate

Src/Particle/AMReX_NeighborList.H

@@ -28,6 +28,9 @@ struct Neighbors
         AMREX_GPU_HOST_DEVICE
         ParticleType& operator* () const { return m_pstruct[m_nbor_list_ptr[m_index]];  }
 
+        AMREX_GPU_HOST_DEVICE
+        unsigned int index () const { return m_nbor_list_ptr[m_index]; }
+
     private:
         int m_index;
         int m_stop;
@@ -51,6 +54,9 @@ struct Neighbors
         AMREX_GPU_HOST_DEVICE
         const ParticleType& operator* () const { return m_pstruct[m_nbor_list_ptr[m_index]];  }
 
+        AMREX_GPU_HOST_DEVICE
+        unsigned int index () const { return m_nbor_list_ptr[m_index]; }
+
     private:
         int m_index;
         int m_stop;