Remove redundant timestep code

[danieljvickers]

Description

I noticed about 300 lines of extremely similar code related to how the RK steps were implemented. I instead created a single subroutine and removed the redundant copies. I also merged two of the parallel loops into one, as there was no race condition created by the two loops

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Something else

Scope

• This PR comprises a set of related changes with a common goal

If you cannot check the above box, please split your PR into multiple PRs that each have a common goal.

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.
Provide instructions so we can reproduce.
Please also list any relevant details for your test configuration

• Ran the test suite called with mfc.sh

Test Configuration:

• What computers and compilers did you use to test this:

Ran on my local machine

Checklist

• I have added comments for the new code
• I added Doxygen docstrings to the new code
• I have made corresponding changes to the documentation (docs/)
• I have added regression tests to the test suite so that people can verify in the future that the feature is behaving as expected
• I have added example cases in examples/ that demonstrate my new feature performing as expected.
  They run to completion and demonstrate "interesting physics"
• I ran ./mfc.sh format before committing my code
• New and existing tests pass locally with my changes, including with GPU capability enabled (both NVIDIA hardware with NVHPC compilers and AMD hardware with CRAY compilers) and disabled
• This PR does not introduce any repeated code (it follows the DRY principle)
• I cannot think of a way to condense this code and reduce any introduced additional line count

If your code changes any code source files (anything in src/simulation)

To make sure the code is performing as expected on GPU devices, I have:

• Checked that the code compiles using NVHPC compilers
• Checked that the code compiles using CRAY compilers
• Ran the code on either V100, A100, or H100 GPUs and ensured the new feature performed as expected (the GPU results match the CPU results)
• Ran the code on MI200+ GPUs and ensure the new features performed as expected (the GPU results match the CPU results)
• Enclosed the new feature via nvtx ranges so that they can be identified in profiles
• Ran a Nsight Systems profile using ./mfc.sh run XXXX --gpu -t simulation --nsys, and have attached the output file (.nsys-rep) and plain text results to this PR
• Ran an Omniperf profile using ./mfc.sh run XXXX --gpu -t simulation --omniperf, and have attached the output file and plain text results to this PR.
• Ran my code using various numbers of different GPUs (1, 2, and 8, for example) in parallel and made sure that the results scale similarly to what happens if you run without the new code/feature

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 43.58%. Comparing base (ba33af4) to head (75d69a2).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/simulation/m_time_steppers.fpp	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #836      +/-   ##
==========================================
+ Coverage   43.24%   43.58%   +0.33%     
==========================================
  Files          68       68              
  Lines       20183    19691     -492     
  Branches     2402     2359      -43     
==========================================
- Hits         8728     8582     -146     
+ Misses       9986     9669     -317     
+ Partials     1469     1440      -29     

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[wilfonba]

Instead of scaler1,...,scalarN, could you use an array to store the coefficients and pass the correct row/column of that array to the s_advance_q_pb_mv subroutine? The array could be allocated and assigned in s_initialize_time_steppers_module. Additionally, I'd suggest the name dst for destination to replace index.

[danieljvickers]

Reverting back a commit since it looks like that small change broke almost every test case

[sbryngelson]

@danieljvickers, this is getting a little silly/comedic. Why don't you move on to a different issue that you feel you're suited for (send me a message of what you think it is, perhaps), and I'll think about this one a bit. This is really an edge case that probably needs a little more thought from our previous snafus with different types of runners (though this one seems especially special AKA stupid)...

You're in a good place to create some containers for MFC. We could try creating those in a few different formats.