Pure OpenFOAM case for a TGV simulation

We need to have a pure OpenFOAM case for a tiny TGV simulation in tests/pure_openfoam_cases/tiny-tgv.

TGV: Taylor Green Vortices (3d periodic, incompressible NS equations, DNS = without turbulent model)

vx = V0 * sin(X / Lx) * cos(Y / Ly) * cos(Z / Lz)
vy = -V0 * cos(X / Lx) * sin(Y / Ly) * cos(Z / Lz)
vz = 0.0

See https://www.grc.nasa.gov/hiocfd/wp-content/uploads/sites/22/case_c3.3.pdf, https://foss.heptapod.net/fluiddyn/fluidsim/-/blob/branch/default/doc/examples/time_stepping/3d/run_simul.py#L188 and https://snek5000.readthedocs.io/en/latest/tuto_tgv.html

Tiny simulation: very small mesh with can be executed in few second.

We need a README file which explains how to run the simulation.