Steady state callback

[LasNikas]

I tested the callback with the pipe_flow_2d.jl example. It terminates after ~0.8sec.
I consider the change in kinetic energy in a 0.002*10 interval.

pp_callback = PostprocessCallback(; dt=0.02, kinetic_energy)

steady_state = SteadyStateCallback(; dt=0.002, interval_size=10, abstol=1.0e-8, reltol=1.0e-6)

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Codecov Report

Attention: Patch coverage is 12.50000% with 63 lines in your changes missing coverage. Please review.

Project coverage is 69.08%. Comparing base (c565f67) to head (a1d69d6).

Files	Patch %	Lines
src/callbacks/steady_state.jl	0.00%	63 Missing ⚠️

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- Coverage   69.92%   69.08%   -0.84%     
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  Lines        5120     5185      +65     
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+ Hits         3580     3582       +2     
- Misses       1540     1603      +63     

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[svchb]

There is one mistake in the condition since you pop/push the current ekin one value is always 0.0 which will effectively reduce the interval size by one.

[efaulhaber]

I find the name a bit misleading. In grid-based methods, when simulating a physical instability arising from a slightly perturbed steady state, you sometimes subtract the right-hand side of the steady state to remove errors.
From this name, I would expect something like this here.

I'll think about other naming suggestions.

[svchb]

ConvergenceConditionCallBack
ConvergenceTerminationCallBack
TerminationConditionCallBack

[LasNikas]

SteadyStateDetector
SteadyStateTerminator

[LasNikas]

This causes a warning message.
A more elegant solution would be to initially (in initialize!) sort the CallbackSet in such a way that the SteadyStateCallback is last.
The set is stored in integrator.opts.callback. However, I have no idea how to check if the actual callback is the last element or not.

Any idea @efaulhaber and @svchb ?

edit: The reason for this is the following.

• CallbackSet(some_callback, steady_state_callback) is totally fine
• CallbackSet(steady_state_callback, some_callback) causes an error message when terminating. ERROR: MethodError: reducing over an empty collection is not allowed; consider supplying init to the reducer

[LasNikas]

Another (simpler) solution:
@suppress integrator.opts.maxiters = integrator.iter

from https://github.com/JuliaIO/Suppressor.jl