Reuse pgm and multigrid

[yhmtsai]

It is just a quick way to reuse the previous coarsening result in multigrid.

[ginkgo-bot]

Error: The following files need to be formatted:

include/ginkgo/core/distributed/preconditioner/schwarz.hpp
include/ginkgo/core/multigrid/multigrid_level.hpp
include/ginkgo/core/multigrid/pgm.hpp
include/ginkgo/core/solver/multigrid.hpp

You can find a formatting patch under Artifacts here or run format! if you have write access to Ginkgo