adding GaussSBP blending coefficients

[jlchan]

Overwrites #1298, which had some issues with spurious commits in the commit history.

This will improve robustness of GaussSBP solvers by modifying the entropy projection when it yields negative densities and pressures for the compressible Euler equations.

[codecov]

Codecov Report

Merging #1299 (de5f642) into main (fa04120) will decrease coverage by 10.66%.
The diff coverage is 0.00%.

❗ Current head de5f642 differs from pull request most recent head 9398b66. Consider uploading reports for the commit 9398b66 to get more accurate results

@@             Coverage Diff             @@
##             main    #1299       +/-   ##
===========================================
- Coverage   92.94%   82.29%   -10.66%     
===========================================
  Files         359      357        -2     
  Lines       29779    29576      -203     
===========================================
- Hits        27677    24337     -3340     
- Misses       2102     5239     +3137     

Flag	Coverage Δ
unittests	82.29% <0.00%> (-10.66%)	⬇️

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

Impacted Files	Coverage Δ
src/Trixi.jl	71.43% <ø> (+3.25%)	⬆️
src/solvers/dgmulti/flux_differencing_gauss_sbp.jl	0.00% <0.00%> (-93.23%)	⬇️

... and 93 files with indirect coverage changes