MueLu: extend region multigrid to two dimensions

Created by: mayrmt

Expectations

Starting from #2262 (closed), extend the recursive region multigrid implementation to two-dimensional problems using MueLu's uncoupled structured aggregation (#2230).

Current Behavior

For one-dimensional problems, we can use a Smoothed-Aggregation Tentative Prolongator and assume a perfect region-to-processor mapping to use MueLu to create hierarchies and run V-cycles that respect region interfaces. However, two- and three-dimensional cases are not covered, yet.

Motivation and Context

Definition of Done

• recursively identify interface GIDs on coarse levels and build coarse level maps in composite, quasiRegional and regional layout
• use uncoupled structured aggregation
• decide on type of prolongator and use it

Possible Solution

• use/extend the strategy of one-dimensional problems for forming coarse level maps (cf. #2262 (closed))
• use MueLu's uncoupled structured aggregation for coarsening (cf. #2230)

Environment

I'll work on this in branch mayrmt-regions-multigrid in my fork and will share via a pull request.

Interested Parties

@lucbv @rstumin