common: KokkosKernels_default_types.hpp missing namespace

[maartenarnst]

The file common/src/KokkosKernels_default_types.hpp defines various type aliases for use across kokkos-kernels. However, this file is missing a namespace, so that these type alias are made in the global scope.

This issue is thus a suggestion to consider introducing a namespace for the default types. A namespace named e.g. KokkosKernels or KokkosKernels::Impl may work, and such a name may limit the changes that may need to be done in the rest of the code.

[maartenarnst]

OK! Thanks @cwpearson !

Also tagging @romintomasetti.