collected small bugfixes and updates

more neighbor list changes, some new options

Kokkos version of class2 bond, angle, dihedral, and improper from Ray Shan

neighbor_kokkos.cpp: Don't call grow() on neighbor lists that are copies

updated documentation and examples for coreshell

Fixed bugs with pppm/gpu when used with compute group/group

USER-DPD: performance optimizations to ssa_update() in fix_shardlow

Kokkos library update

This corresponds to a bugfix from commit 9161bd98 on neighbor.cpp

Overall improvements range from 2% to 18% on our benchmarks
1) Newton has to be turned on for SSA, so remove those conditionals
2) Rework the math in ssa_update() to eliminate many ops and temporaries
3) Split ssa_update() into two versions, based on DPD vs. DPDE
4) Reorder code in ssa_update_*() to reduce register pressure

simplify FFT3d code by removing support for outdated FFT libraries

Small updates and fixes

Updated polarizer.py in USER-DRUDE to use coul/long/cs

(cherry picked from commit 6410797697ddffbd5b47f526dcabd74f67ff6108)