Initial bare-bones port of FixRX to Kokkos.

Initial port of USER-DPD/fix_rx.cpp to KOKKOS/fix_rx_kokkos.cpp.
Using parallel_reduce(...) but still using host-only data.
TODO:

1. Switch to KOKKOS datatypes for sparse-kinetics data; dense is finished.
2. Switch to using KOKKOS data for dvector.
3. Remove dependencies in rhs(...) on atom. Store those consts in UserData{} or as member constants.
4. Port ComputeLocalTemp(...) to Kokkos (needs pairing algorithm).