USER-DPD: bugfix for AtomVecDPD::unpack_restart() and the Kokkos version

Also, cleaned up UCG_MASK and UCGNEW_MASK usage in atom_vec_dpd_kokkos.cpp
Fixes a crash when using the python interface on a 2nd LAMMPS invocation
when using the restart (or replicate) commands, because of garbage
in the uCG and uCGnew arrays leftover from the previous LAMMPS invocation.