Add a two-level 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.

[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.

[yhmtsai]

Suggested change

	if (cweight > 0.0 && cweight <= 1.0) {
	if (cweight >= 0.0 && cweight <= 1.0) {

[yhmtsai]

throw an error or assert if coarse_level_ or coarse_solver are nullptr.
Do you need the fallback option?
For example, the problem is small enough or different reason such that multigrid level does not generate proper one.
User seems to easily have a wrong expectation. we do not perform the two-level schwarz.

[pratikvn]

I would say that is out of scope of what Schwarz should do. I think the user has to ensure that the coarse level has valid inputs. The fallback option will be that there is no coarse level solve.

[yhmtsai]

you need to wrap if (this->local_solver) to ensure it is not nullptr

[sonarqubecloud]

Quality Gate passed

Issues
7 New issues
0 Accepted issues

Measures
0 Security Hotspots
85.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

Attention: Patch coverage is 97.61905% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.36%. Comparing base (fc280ab) to head (d6c3ebc).
Report is 23 commits behind head on develop.

Files with missing lines	Patch %	Lines
core/distributed/preconditioner/schwarz.cpp	94.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1431      +/-   ##
===========================================
- Coverage    89.57%   89.36%   -0.21%     
===========================================
  Files          824      824              
  Lines        68815    68983     +168     
===========================================
+ Hits         61643    61649       +6     
- Misses        7172     7334     +162     

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