Add a Multilevel schwarz preconditioner

[pratikvn]

This PR uses the distributed coarse level generation from PGM to use as a coarse level for the additive Schwarz preconditioner. The only requirement is that the galerkin product generator ($RAP$) generate a multigrid::MultigridLevel which is a triplet of restrict, prolong and coarse operators. The user additionally also needs to set the solver for the coarse level.

TODO

• Add options to switch between multiplicative and additive
• Options for arbitrary number of levels.

Possible issues

1. The coarse level solve significantly reduces the number of iterations, but can be extremely expensive. Maybe makes sense to apply coarse solve, only a few times instead of every preconditioner apply (need to be careful of consistency of preconditioner apply) ?
2. Weighting between the local solver solution and the coarse solution is unclear.
3. Though the coarse solve and the local solve are independent, we have no way of performing them in parallel. This will probably require a rewrite of the apply interface to enable asynchronicity, in addition to support for multiple streams/queues.

[ginkgo-bot]

Error: The following files need to be formatted:

core/distributed/preconditioner/schwarz.cpp
core/test/mpi/distributed/preconditioner/schwarz.cpp
examples/distributed-solver/distributed-solver.cpp
include/ginkgo/core/distributed/preconditioner/schwarz.hpp
test/mpi/preconditioner/schwarz.cpp

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

[pratikvn]

As a first step, I just added a two-level preconditioner with equal weighting for the local solution and the coarse solution. I think it maybe makes sense to try this out in some applications and then think about arbitrary number of levels, additive/multiplicative etc.

[pratikvn]

format!