(Thread-)Parallelize bounds check routine for subcell IDP limiting

[bennibolm]

For larger simulations, the bounds check routine for IDP limiting requires more and more time since it was not parallelized. After this PR, the loop over all elements is thread-parallelized.
For that, the deviation memory had to be changed:
I divided it into its global idp_bounds_delta_global and local component idp_bounds_delta_local - both are Dictionaries for the respective variable bounds.

• idp_bounds_delta_local (containing the maximum deviations in the current timestep interval) is now a vector and therefore parallel-safe. Due to false sharing we extend the vector and use a stride size.

• idp_bounds_delta_global (containing the global maximum deviations) doesn't need to be thread-safe, since it only uses the already parallel-computed result of the local maximum deviation.

Additionally, this PR parallelizes resetting the subcell limiting coefficients alpha.

[github-actions]

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[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (fcf2652) 96.34% compared to head (9c66099) 96.34%.

Files	Patch %	Lines
src/callbacks_stage/subcell_bounds_check_2d.jl	96.88%	1 Missing ⚠️

Additional details and impacted files

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##             main    #1736      +/-   ##
==========================================
- Coverage   96.34%   96.34%   -0.00%     
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  Files         451      451              
  Lines       35979    35996      +17     
==========================================
+ Hits        34662    34677      +15     
- Misses       1317     1319       +2     

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[bennibolm]

EDIT: Outdated structure

The current thread implementation is not the cleanest.

    # Threaded memory for bounds checking routine with `BoundsCheckCallback`.
    # The first entry of each vector contains the maximum deviation since the last export.
    # In the second entry, the total maximum deviation is saved.
    idp_bounds_delta_threaded = Dict{Symbol, Vector{Vector{real(basis)}}}()
    for key in bound_keys
        idp_bounds_delta_threaded[key] = [zeros(real(basis), 2)
                                          for _ in 1:Threads.nthreads()]
    end

First, the "Dictionary-Layer" has to be first, since the call of Symbol("...") always creates a small amount of allocations.
I'm aware that the memory for the global maximum deviation for threads 2 to nthreads() aren't used right now. Maybe there is another way to implement this thread parallel bounds check.
(Idea: Use Dictionary with vectors of length nthreads() + 1. Use the first nthreads() entries are used to calculate the current deviations in parallel. Then, use last entry for global deviation, which is not needed to be parallel. This would save memory, but probably would confuse more....)

EDIT: Outdated structure

[efaulhaber]

I don't like this data structure, but I also don't have any better ideas.

[bennibolm]

In the last commit (78957b7), I revised the memory structure for the IDP bounds check.
For a better understanding, the local and global component of the memory are now divided into 2 different variable. The local component needed to be thread-parallel-safe and is therefore initialized as a vector of length nthreads(). On the other hand, this was not required for the global maximum deviation component.

[sloede]

One minor comment left, then go go go!

[sloede]

LGTM, thanks!