Add sized ctor and public add_operator to Combination

[greole]

This PR makes the Combination::add_operators member public so that operators can be added after constructing a combination matrix. Additionally, a new constructor is added so that an empty combination can be created first and additional operators are added later.

[upsj]

Can you describe a use case for this? Combination creation is pretty lightweight, so I am hesitant to add more complexity there.

[greole]

Sure and maybe there is a better way to do it. On the OGL side I have std::vector<gko::LinOp> of local and non_local linops representing local and non_local interfaces for Distributed::Matrix. Probably, I could also use this ctr

    explicit Combination(CoefficientIterator coefficient_begin,
                         CoefficientIterator coefficient_end,
                         OperatorIterator operator_begin,
                         OperatorIterator operator_end)

but there are cases (after repartitioning) where operator_begin==operator_end thus it would throw.

[upsj]

Making the empty case work seems sensible, except for the fact that we don't have anything to base the size on then. BTW ctr is kind of a misleading abbreviation, it could be counter or constructor.

[MarcelKoch]

I agree with @upsj that we should not expose this member. I think it would be enough to add a constructor, which takes the iterators, and just an dimension as an extra parameter. Then you could build the linop from that parameter, and just check that the operator dimensions match up.

[ginkgo-bot]

Error: The following files need to be formatted:

include/ginkgo/core/base/combination.hpp
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