Some multi-threading improvements

[ranocha]

I improved multi-threading a bit

• Fix multi-threading for parabolic terms on ARM (such as Apple M series processors)
• Reduce allocations a bit

See the commit messages for more details.

[github-actions]

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[sloede]

Nice work, thanks!

[sloede]

(you need to run the formatter)

[codecov]

Codecov Report

Patch coverage: 93.22% and project coverage change: -5.02% ⚠️

Comparison is base (953f88a) 96.14% compared to head (c217951) 91.13%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1630      +/-   ##
==========================================
- Coverage   96.14%   91.13%   -5.02%     
==========================================
  Files         414      414              
  Lines       33942    33943       +1     
==========================================
- Hits        32633    30931    -1702     
- Misses       1309     3012    +1703     

Flag	Coverage Δ
unittests	91.13% <93.22%> (-5.02%)	⬇️

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

Files Changed	Coverage Δ
src/solvers/dgsem_tree/dg_3d_parabolic.jl	92.65% <82.61%> (+0.07%)	⬆️
src/solvers/dgmulti/dg_parabolic.jl	90.34% <100.00%> (+0.05%)	⬆️
src/solvers/dgsem_tree/dg_1d_parabolic.jl	94.44% <100.00%> (+0.13%)	⬆️
src/solvers/dgsem_tree/dg_2d_parabolic.jl	95.95% <100.00%> (+0.04%)	⬆️

... and 56 files with indirect coverage changes

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