improve performance of kernel(::SchoenbergQuarticSplineKernel, r, h)

[ranocha]

Continuing a discussion with @efaulhaber on Slack - here is the code of kernel7 I mentioned on Slack. Feel free to use this and adapt the other kernels accordingly.

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (b6f8710) 57.28% compared to head (facba89) 57.28%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #253   +/-   ##
=======================================
  Coverage   57.28%   57.28%           
=======================================
  Files          55       55           
  Lines        2943     2943           
=======================================
  Hits         1686     1686           
  Misses       1257     1257           

Flag	Coverage Δ
unit	57.28% <65.51%> (ø)

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Files	Coverage Δ
src/TrixiParticles.jl	100.00% <ø> (ø)
src/schemes/solid/total_lagrangian_sph/system.jl	75.43% <100.00%> (-1.93%)	⬇️
src/general/corrections.jl	29.87% <94.73%> (+21.24%)	⬆️
src/general/smoothing_kernels.jl	30.26% <0.00%> (ø)

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[ranocha]

As I said on Slack, this gives slightly different results since it uses another algorithm that is slightly less accurate but much faster. I leave it up to you to evaluate it for your needs - and adapt other parts accordingly.

I do not understand the comment above completely - and I don't know which version of the other PR you have used. Anyway, if you include optimizations like the ones I suggested here, I strongly suggest to include the comments explaining why these changes are made as well.

[svchb]

Closed as the other PR has been merged.