Remove NNP code and move to n2p2

[streeve]

Steps towards removing the n2p2 specific code:

• [x] Merge flexible AoSoA #30, templating over System and System_NNP (better interface to main n2p2 kernels)
• [x] Merge neighbor list update #43, templating over Neighbor
• [x] Merge update for NNP kernels 46 to use Cabana slices, rather than CabanaMD System
• [x] Merge template over device type #51 and clean up use of Kokkos Views for NNP
• [x] Merge into n2p2
• [x] Remove all NNP files except force_nnp* (wrapping n2p2 functions)