Add shock flattener

[pgrete]

@BenWibking reported on https://gitlab.com/theias/hpc/jmstone/athena-parthenon/athenapk/-/issues/14:

It would be very useful to implement the shock flattener from Miller & Colella (2002): https://ui.adsabs.harvard.edu/abs/2002JCoPh.183...26M/abstract

Thread:

@pgrete FYI (not sure if you've seen it), first order flux correction is implemented to fix troubled cells.
@BenWibking Yes, I've seen that. In my tests with Quokka, the shock flattener appeared necessary to suppress some post-shock oscillations in strong shocks. I remember this being a particular issue with the Zel'dovich spike in radiative shocks. Do you see any of these kinds of issues currently? I didn't do characteristic reconstruction in Quokka, so that might be a difference.

@pgrete I see. I didn't think of the post-shock oscillations. For those the flattener might be the better choice.

@BenWibking Is first-order flux correction implemented for RK2/RK3?

@pgrete We call first order flux correction in each stage at the moment (so not just at the very end). I'm not sure which is the better approach, but I went for this one as I think that "each stage" might be better because no neighboring cells will be updated during an entire cycle with potentially nonphysical data from a previous stage.
@BenWibking That's what I did in Quokka as well. If I didn't do this at the end of each stage, then in the later stages, the Riemann solver would produce NANs from the imaginary sound speeds. So this makes sense to me.

@BenWibking The oscillations can also be reproduced with a pure-hydro shock tube, such as this one: https://github.com/BenWibking/quokka/blob/development/src/HydroShocktube/test_hydro_shocktube.cpp based on this test problem: https://cococubed.com/code_pages/ppm_1d.shtml

[BenWibking]

For reference, the Quokka implementation is here: https://github.com/quokka-astro/quokka/blob/4c34ac8fd1299192ef8934b7959eb73f83b27bea/src/hydro_system.hpp#L441