[WIP] Flux agnostic @generated 3D nonconservative volume turbo kernel

[MarcoArtiano]

The generated code mimics the optimized hand-written code in trixi-framework/TrixiAtmo.jl#128.

Nonconservative terms benchmarks

MHD with nonconservative terms for p4est_3d_dgsem/elixir_mhd_alfven_wave_nonperiodic.jl
Plain implementation volume_flux = (flux_hindenlang_gassner, flux_nonconservative_powell)

BenchmarkTools.Trial: 4 samples with 1 evaluation per sample.
 Range (min … max):  1.318 s …   1.366 s  ┊ GC (min … max): 0.00% … 2.25%
 Time  (median):     1.324 s              ┊ GC (median):    0.00%
 Time  (mean ± σ):   1.333 s ± 22.366 ms  ┊ GC (mean ± σ):  0.58% ± 1.13%

  ██           █                                          █
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  1.32 s         Histogram: frequency by time        1.37 s <

 Memory estimate: 33.32 MiB, allocs estimate: 9460.

FluxVolumeTurbo(flux_hindenlang_gassner, flux_nonconservative_powell) this does not precompute the primitive variables.

BenchmarkTools.Trial: 6 samples with 1 evaluation per sample.
 Range (min … max):  982.983 ms …   1.035 s  ┊ GC (min … max): 0.00% … 2.90%
 Time  (median):     984.219 ms              ┊ GC (median):    0.00%
 Time  (mean ± σ):   993.311 ms ± 20.522 ms  ┊ GC (mean ± σ):  0.50% ± 1.19%

  ██      ▁                                                  ▁
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  983 ms          Histogram: frequency by time          1.03 s <

 Memory estimate: 33.35 MiB, allocs estimate: 9512.

Combined approach in p4est_3d_dgsem/elixir_mhd_alfven_wave_combined_fluxes_nonperiodic.jl

BenchmarkTools.Trial: 4 samples with 1 evaluation per sample.
 Range (min … max):  1.226 s …    1.565 s  ┊ GC (min … max): 0.00% … 0.00%
 Time  (median):     1.231 s               ┊ GC (median):    0.00%
 Time  (mean ± σ):   1.313 s ± 167.761 ms  ┊ GC (mean ± σ):  0.11% ± 0.23%

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  1.23 s         Histogram: frequency by time         1.56 s <

 Memory estimate: 33.32 MiB, allocs estimate: 9446.

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