Remove openmp version guard in vtkm and add precision options (#10005)
Remove openmp version guard in vtkm and add precision options (#10005)

Clang 8.0 now supports openmp 4.5 and many commerical compilers also support
openmp >= 4.0. This commit removes the openmp version guard and rely on CMake
to throw errors out when openmp version requirement is not satisified.

It also add options for precision control.
iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack