Add tridiagonal solver design document

[mwarusz]

Checklist

• Documentation:
  • Design document has been generated and added to the docs
  • User's Guide has been updated
  • Developer's Guide has been updated
  • Documentation has been built locally and changes look as expected
• Testing
  • A comment in the PR documents testing used to verify the changes including any tests that are added/modified/impacted.
  • CTest unit tests for new features have been added per the approved design.
  • Polaris tests for new features have been added per the approved design (and included in a test suite)
  • Unit tests have passed. Please provide a relevant CDash build entry for verification.
  • Polaris test suite has passed
  • Performance related PRs: Please include a relevant PACE experiment link documenting performance before and after.
• Stealth Features
  • If any stealth features are included in the PR, please confirm that they have been documented.

[cbegeman]

Note that this design doc includes the Schopf and Loughe (1995) approach for accuracy and stability discussed here: E3SM-Ocean-Discussion#42

[mwarusz]

@mark-petersen

I have updated this design doc and it is now ready for review. I also opened a draft PR with the implementation (#194). When reviewing the design it might be helpful for you to have a look at one of the unit tests showing how these solvers are used to solve a simple 1D diffusion problem:

https://github.com/mwarusz/E3SM/blob/5617516be139facfaa904e0713287a0d9e4d5b8f/components/omega/test/base/TridiagonalTest.cpp#L146-L245