WIP: Custom Integrators: Add support for save_everystep==true

[bennibolm]

For the existing custom integrators, the parameter save_everystep has no significance. I changed that exemplary for SimpleIntegratorSSP and would add it to SimpleIntegrator2N and SimpleIntegrator3Sstar if desired.

[github-actions]

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[bennibolm]

I never really used save_everystep=true, but thought it would be good to be consistent with its functionality in OrdinaryDiffEq. What do you think? @ranocha

[sloede]

I never really used save_everystep=true, but thought it would be good to be consistent with its functionality in OrdinaryDiffEq. What do you think? @ranocha

I don't have a strong opinion here but I propose to follow YAGNI: do not implement anything you do not need, since every unused feature costs time during development, testing, maintenance, and understanding the code base.

If you want to avoid silent errors, I would find it more suitable to throw an error that says that this feature is not implemented.

Then again, I have no strong opinion here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e900262) 82.10% compared to head (c3cc65e) 96.10%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1689       +/-   ##
===========================================
+ Coverage   82.10%   96.10%   +13.99%     
===========================================
  Files         425      425               
  Lines       34382    34410       +28     
===========================================
+ Hits        28229    33067     +4838     
+ Misses       6153     1343     -4810     

Flag	Coverage Δ
unittests	96.10% <100.00%> (+13.99%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
src/time_integration/methods_SSP.jl	83.87% <100.00%> (+83.87%)	⬆️

... and 107 files with indirect coverage changes

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[ranocha]

I basically agree with @sloede - if you don't need it, you shouldn't implement it. Note that it is quite easy to blow up your memory if you use save_everystep = true for a 3D simulation...

[bennibolm]

Okay, thank you both. I'll just keep the branch for the case I need it someday, and close the PR.