Remove RCM Matrix Reordering from Direct Solver

[julianlitz]

This PR proposes removing the Reverse Cuthill–McKee (RCM) matrix reordering step from the direct solver.

• Maintenance Burden: Extra code paths that need to be maintained and tested
• GPU Incompatibility: RCM implementation relies on std::queue, which might be problematic for GPU execution

Arguments against removal:

• Increased fill-in: 25.6% more fill-in elements
• Slower setup: 2x slower factorization (though the direct solver isn't really the bottleneck)

Merge Request - GuideLine Checklist

Guideline to check code before resolve WIP and approval, respectively.
As many checkboxes as possible should be ticked.

Checks by code author:

Always to be checked:

• There is at least one issue associated with the pull request.
• New code adheres with the coding guidelines
• No large data files have been added to the repository. Maximum size for files should be of the order of KB not MB. In particular avoid adding of pdf, word, or other files that cannot be change-tracked correctly by git.

If functions were changed or functionality was added:

• Tests for new functionality has been added
• A local test was succesful

If new functionality was added:

• There is appropriate documentation of your work. (use doxygen style comments)

If new third party software is used:

• Did you pay attention to its license? Please remember to add it to the wiki after successful merging.

If new mathematical methods or epidemiological terms are used:

• Are new methods referenced? Did you provide further documentation?

Checks by code reviewer(s):

• Is the code clean of development artifacts e.g., unnecessary comments, prints, ...
• The ticket goals for each associated issue are reached or problems are clearly addressed (i.e., a new issue was introduced).
• There are appropriate unit tests and they pass.
• The git history is clean and linearized for the merge request. All reviewers should squash commits and write a simple and meaningful commit message.
• Coverage report for new code is acceptable.
• No large data files have been added to the repository. Maximum size for files should be of the order of KB not MB. In particular avoid adding of pdf, word, or other files that cannot be change-tracked correctly by git.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.02%. Comparing base (6c8ca13) to head (fbe61a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
==========================================
- Coverage   88.21%   88.02%   -0.20%     
==========================================
  Files          89       89              
  Lines        5246     5146     -100     
==========================================
- Hits         4628     4530      -98     
+ Misses        618      616       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[EmilyBourne]

I don't think I should be the one to decide if functionalities should be removed.

For the effect on GPU, I imagine the algorithm with std::queue could be rewritten slightly to be GPU compatible if the GMGPolar team prefer to keep it.
Alternatively, maybe there is a way to refactor so RCM matrix reordering can be activated or deactivated.