SUNStepper basics based on MRIStepInnerStepper

[balos1]

I am opening this PR as a draft to gather feedback. This renames/moves MRIStepInnerStepper to SUNStepper so that @Steven-Roberts and I can extend it for other purposes.

[balos1]

@drreynolds @gardner48 I have made the changes we discussed on Tuesday. Let me know what you think.

[drreynolds]

I like how clean this is now. I noticed a few items for comment (see below).

[Steven-Roberts]

Initial pass at latest changes

[Steven-Roberts]

After some more thought and discussion with David on IDA and passing around yp, the primary issue I see is keeping it consistent with y. With operator splitting, for example, y jumps around between calls to evolve, and I don't see how to keep yp consistent. With MRI, the change in the forcing between stages can cause a similar issue. This is not something the outer methods can do and needs to be done in the inner SunStepper integrator.

I'd like to propose the following to make SunStepper self starting (all you need is y to reset then evolve):

• Remove yp as arguments
• To support an IDA SunStepper, have a constructor like

int IDACreateSUNStepper(void* inner_ida_mem, ConsistentYPrimeFn f, SUNStepper* stepper)

where there's a function which can be specified by users (we could have a default that uses IDACalcIC) to get yp given y.

[balos1]

I actually need the OneStep function in the adjoint work, so I have added it back into here.

[drreynolds]

It looks like the only remaining issue with the CI is that the documentation updates have some label errors:

/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/RecentChanges.rst:5: WARNING: undefined label: 'arkode.usage.splittingstep'
/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/RecentChanges.rst:5: WARNING: undefined label: 'arkode.usage.forcingstep'
/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/sunstepper/SUNStepper_Implementing.rst:42: WARNING: c:func reference target not found: SplittingStepCreate
/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/sunstepper/SUNStepper_Implementing.rst:42: WARNING: c:func reference target not found: ForcingStepCreate
/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/sunstepper/SUNStepper_Implementing.rst:55: WARNING: c:func reference target not found: SplittingStepCreate
/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/sunstepper/SUNStepper_Implementing.rst:55: WARNING: c:func reference target not found: ForcingStepCreate
/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/RecentChanges.rst:5: WARNING: undefined label: 'arkode.usage.splittingstep'
/var/lib/jenkins/workspace/SUNDIALS_PR-463/doc/shared/RecentChanges.rst:5: WARNING: undefined label: 'arkode.usage.forcingstep'

[Steven-Roberts]

Yeah, wasn't sure how to best handle that. Those are references to parts of the docs that will be added in the operator splitting PR (which is merged after this). I can temporarily comment out or remove the references out to get the CI passing.

[drreynolds]

Yeah, wasn't sure how to best handle that. Those are references to parts of the docs that will be added in the operator splitting PR (which is merged after this). I can temporarily comment out or remove the references out to get the CI passing.

I forget, but is the "merge" plan to approve/merge this PR before #572? If that's the case, then perhaps we should move these bits of documentation into #572 so that both "squashed" and merged PRs can pass all CI tests?

[Steven-Roberts]

I forget, but is the "merge" plan to approve/merge this PR before #572?

Yes

If that's the case, then perhaps we should move these bits of documentation into #572 so that both "squashed" and merged PRs can pass all CI tests?

Ok, I'll do that

[gardner48]

One minor change (missing process error call) that I'll apply and approve in a sec.