add optional Kokkos support to magistrate

[cz4rs]

distBVH uses spack packages in CI and it fails when attempting to serialize Kokkos::View:

/opt/view/include/checkpoint/dispatch/dispatch_serializer_nonbyte.h:233:16: error: no matching function for call to 'checkpoint::dispatch::SerializerDispatchNonByte<checkpoint::Sizer, Kokkos::View<unsigned char*, Kokkos::LayoutRight, Kokkos::OpenMP>, checkpoint::dispatch::BasicDispatcher<checkpoint::Sizer, Kokkos::View<unsigned char*, Kokkos::LayoutRight, Kokkos::OpenMP> > >::applyStatic(checkpoint::Sizer&, Kokkos::View<unsigned char*, Kokkos::LayoutRight, Kokkos::OpenMP>*&, checkpoint::SerialSizeType&)'

TODO:
Add Kokkos as an optional dependency and corresponding variant (default=False) to magistrate package.

magistrate enables Kokkos support based on existence of Kokkos_DIR or Kokkos_ROOT, so I'm assuming something like "-DKokkos_ROOT={}".format(self.spec["kokkos"].prefix) will be necessary.