Switch (back) to Einfeldt wave-speed estimate from paper

[DanielDoehring]

Due to the breaking change in #1708
the wave speed estimate now being used ("true" naive) in the 1D HLLC flux for MHD is actually not the desired one (Einfeldt).
This change takes care of this.

[github-actions]

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[ranocha]

Thanks!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (12b0839) 93.53% compared to head (4419f3f) 93.53%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1751   +/-   ##
=======================================
  Coverage   93.53%   93.53%           
=======================================
  Files         431      431           
  Lines       34853    34853           
=======================================
  Hits        32598    32598           
  Misses       2255     2255           

Flag	Coverage Δ
unittests	93.53% <100.00%> (ø)

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[andrewwinters5000]

LGTM!

[ranocha]

Downstream CI fails due to JuliaVTK/ReadVTK.jl#49. I'll merge this PR