Rayleigh--Taylor Instabilty (ideal gas, equal mass, 3D)
--------------

This is a 3D version of the Rayleigh--Taylor instability. These initial
conditions are those used to produce the simulations presented by
Sandnes et al. (2025), section 4.5.

This test uses particles of equal mass and has a sharp density.

Note that the default resolution_eta parameter is consistent with the use of a
Wendland C2 kernel with ~100 neighbours.

Some examples of configuration options with different hydro schemes:

REMIX:
`--with-hydro=remix --with-equation-of-state=planetary --with-kernel=wendland-C2 --with-ext-potential=constant --with-adiabatic-index=7/5 --enable-boundary-particles=20993`

"Traditional" SPH (tSPH):
`--with-hydro=planetary --with-equation-of-state=planetary --with-kernel=wendland-C2 --with-ext-potential=constant --with-adiabatic-index=7/5 --enable-boundary-particles=20993`