make shock tube and blast wave tests run for SRHD when using MDA AV

The key step was to adapt a 'covariant' regularization that looks like

\partial_t D + \partial_x f_D^x = - \partial_x ( \mu ( \partial_t - \partial_x ) D ),

where the time derivative on the RHS is new compared to previous attempts. We approximate \partial_t D by the - \partial_x f_D^x.

Note that this is not truely covariant, but (\partial_t - \partial_x) ( \mu ( \partial_t - \partial_x ) D) would be. Problem is that we don't know how to approximate the second time derivatives or the mixed time-space derivatives. But it works this way, so leave it for now.

This PR adds example parameter files that allow to reproduce the SRHD tests from https://arxiv.org/pdf/2202.08839.pdf. To run the blastwave2 test it was important to add correct the evolved variable tau whenever the energy density eps turned slightly negative, as suggested in https://arxiv.org/pdf/2005.01821.pdf III.A. What is interesting is that atm we don't need an atmosphere description, but that's probably because those tests do not examine low densities.

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Side note: Realized that !75 (merged) promoted Mesh to a parameteric type and now most of the HRSC and TCI structs suffer from type instabilities. Need to fix this in a separate PR.

examples/simple_wave/aventropy.toml

+[EquationOfState]
+eos = "polytrope"
+polytrope_gamma = 1.6666666666666666667
+polytrope_k = 100.0
+
+[SRHD]
+id = "simple_wave"
+bc = "from_id"
+
+[Mesh]
+range = [ -1.5, 1.5 ]
+n = 5
+k = 125
+basis = "lgl"
+periodic = true
+
+[Output]
+variables       = [ "D", "S", "tau", "p", "rho", "eps", "v", "EP", "smoothed_mu", "E", "flx_E" ]
+# variables       = [ "D", "S", "tau" ]
+aligned_ts      = "$(collect(range(0.01,6.4,step=0.01)))"
+enable1d        = true
+
+[Evolution]
+cfl = 0.125
+tend = 6.4
+# tend = 0.2
+
+[Log]
+progress_stdout = true
+
+[HRSC]
+method = "av"
+av_method = "entropy"
+entropy_cmax = 1.0
+entropy_ce   = 1.0
+
+[TCI]
+method = "entropy_production"