Algebraic multigrid preconditioner changes

Purpose

I made some changes to algebraic multigrid preconditioner to make it more useable: 1) Renamed AGMG to AMG to avoid confusion with agglomerated multigrid methods like in #49 2) Added separate AMG options for the ANK, NK, and adjoint solvers. I made this change because I had cases where AMG was effective for the adjoint but not ANK. This is a breaking change because option names and behavior are different. 3) Fixed a bug where AMG would crash when using 1 outer iteration 4) Added the ability to use multiple ILU iterations in the AMG smoother. This maintains consistency with the ASM+ILU combination we use when not using AMG.

This PR could close #49 depending on how much more documentation and testing we want.

Expected time until merged

3 weeks

Type of change

[x] Bugfix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[x] Breaking change (non-backwards-compatible fix or feature)
[x] Code style update (formatting, renaming)
[ ] Refactoring (no functional changes, no API changes)
[ ] Documentation update
[ ] Maintenance update
[ ] Other (please describe)

Testing

I added tests for different combinations of AMG levels and iterations, including a test for the bugfix when using 1 outer iteration.

Checklist

[x] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
[x] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
[x] I have run unit and regression tests which pass locally with my changes
[x] I have added new tests that prove my fix is effective or that my feature works
[x] I have added necessary documentation

mdolab / adflow