Enable optional installation of Kokkos

[lsawade]

Task description

For testing of Kokkos versions it would be good to allow Our configuration to try and find a local installation, or set something like SPECFEM_KOKKOS_FORCE_INSTALL. This would drastically simplify testing Kokkos branches especially when thinking about updgrading to Kokkos 5.0, see #1330 .

Acceptance criteria

• Local Kokkos can be used
• tests still pass