Store matrices in LDU format to improve performance of LU-SGS and ILU preconditioners

[pcarruscag]

Proposed Changes

Store matrices in LDU format
On a representative 3D case (one of the HLCRM meshes):
Time per iteration with ILU: 2.65s -> 2.4s
Time per iteration with LU_SGS: 2.05s -> 1.85s

I was not very careful in making sure these profiles were 100% apples to apples by covering the same number of iterations but they show the preconditioner time is not much closer to the matrix-vector product.
ILU develop:

ILU using LDU format

SGS develop:

SGS using LDU format:

Related Work

Closes #2837

PR Checklist

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with --warnlevel=3 when using meson).
• My contribution is commented and consistent with SU2 style (https://su2code.github.io/docs_v7/Style-Guide/).
• I used the pre-commit hook to prevent dirty commits and used pre-commit run --all to format old commits.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp), if necessary.