Adds (S)SOR Preconditioner

[MarcelKoch]

This PR adds the core and reference implementation of a (S)SOR preconditioner. It uses triangular solves to apply the preconditioner. The preconditioner is either a single lower triangular solver, or a lower triangular solve followed by an upper triangular one. Thus the preconditioner is represented as a gko::Combination. The Sor class itself is only a linop factory, similar to the Lu factorization. It can be parameterized by the relaxation_factor (0 < w < 2), symmetric or not (sym version has the two triangular solves), and the solver factories for the upper and lower triangular matrices.

This also adds a Gauss-Seidel preconditioner as a special case of SOR.

Todo:

• documentation
• wait for Require C++17 support #1603

[yhmtsai]

LGTM in general.
I am thinking of how to add the test for GaussSeidel in a cheap way or just do not test it, which calls SOR with relaxation 1

[yhmtsai]

LGTM. The config test from registry is not yet there

[fritzgoebel]

LGTM, only a few small comments / questions.

Note: I did not look at the correctness of the config parsing.

[fritzgoebel]

LGTM, only a few small comments / questions.

Note: I have not looked at the correctness of the config parsing.

[codecov]

Codecov Report

Attention: Patch coverage is 97.31183% with 10 lines in your changes missing coverage. Please review.

Project coverage is 90.33%. Comparing base (30c2d86) to head (4b83873).
Report is 9 commits behind head on develop.

Files with missing lines	Patch %	Lines
common/unified/preconditioner/sor_kernels.cpp	0.00%	4 Missing ⚠️
core/device_hooks/common_kernels.inc.cpp	0.00%	2 Missing ⚠️
include/ginkgo/core/preconditioner/sor.hpp	81.81%	2 Missing ⚠️
...nclude/ginkgo/core/preconditioner/gauss_seidel.hpp	90.00%	1 Missing ⚠️
reference/factorization/factorization_kernels.cpp	90.90%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1633      +/-   ##
===========================================
- Coverage    92.05%   90.33%   -1.73%     
===========================================
  Files          764      776      +12     
  Lines        62678    63228     +550     
===========================================
- Hits         57701    57116     -585     
- Misses        4977     6112    +1135     

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[sonarcloud]

Quality Gate passed

Issues
29 New issues
0 Accepted issues

Measures
0 Security Hotspots
93.5% Coverage on New Code
6.5% Duplication on New Code

See analysis details on SonarCloud