avoid tuncation error through integer division and promote to use "bigint" instead of "int" to avoid overflows for large systems

Patch release 15 November 2018

Collected small changes for next release

OpenMP support from the compiler is not a requirement for USER-OMP. Without OpenMP, it is like the OPT package but for many more styles, so it is still useful and should be supported.

Linkage mode improvements and documentation updates for USER-PLUMED package

cmake: update internal doc about how cmake finds executables

add detailed build instructions and discussion of linkage modes for PLUMED library and USER-PLUMED package

support all three plumed linkage modes with CMake as well. For downloaded and previously installed plumed lib

add code and scripts to support all three plumed linkage modes with fix plumed for conventional build

Collection of small changes and bugfixes for the next release

Add Hyper-dynamics to REPLICA package

Update Kokkos library in LAMMPS to v2.7.24