update kokkos compilation instructions to use provided preset makefiles

cd lammps/src

make yes-kokkos

make g++ KOKKOS_DEVICES=OpenMP :pre

make kokkos_omp :pre

Intel Xeon Phi:

CPU-only (only MPI, no threading):

cd lammps/src

make yes-kokkos

make g++ KOKKOS_DEVICES=OpenMP KOKKOS_ARCH=KNC :pre

make kokkos_mpi :pre

CPUs and GPUs:

Intel Xeon Phi (Intel Compiler, Intel MPI):

cd lammps/src

make yes-kokkos

make cuda KOKKOS_DEVICES=Cuda :pre

make kokkos_phi :pre

CPUs and GPUs (with MPICH):

cd lammps/src

make yes-kokkos

make kokkos_cuda_mpich :pre

These examples set the KOKKOS-specific OMP, MIC, CUDA variables on the

make command line which requires a GNU-compatible make command.  Try

or "make clean-machine" before each build.  This is to force all the

KOKKOS-dependent files to be re-compiled with the new options.

You can also hardwire these make variables in the specified machine

makefile, e.g. src/MAKE/Makefile.g++ in the first two examples above,

with a line like:

KOKKOS_ARCH = KNC :pre

Note that if you build LAMMPS multiple times in this manner, using

different KOKKOS options (defined in different machine makefiles), you

do not have to worry about doing a "clean" in between.  This is

because the targets will be different.

NOTE: The 3rd example above for a GPU, uses a different machine

makefile, in this case src/MAKE/Makefile.cuda, which is included in

the LAMMPS distribution.  To build the KOKKOS package for a GPU, this

makefile must use the NVIDA "nvcc" compiler.  And it must have a

KOKKOS_ARCH setting that is appropriate for your NVIDIA hardware and

installed software.  Typical values for KOKKOS_ARCH are given below,

as well as other settings that must be included in the machine

makefile, if you create your own.

NOTE: Currently, there are no precision options with the KOKKOS

package.  All compilation and computation is performed in double

precision.