Get unit testing back

[mkruckow]

We are saying for a long time, that we'd like to get the unit testing back.

Here we may want to have a list of stuff to be done for this (I'll start):

• there is no test grid: posydon/tests/data/POSYDON-UNIT-TESTS/visualization/grid_unit_test_plot.h5
  This causes a ValueError: Test grid for unit testing was not found! when trying to do either of the following imports:

import posydon.tests.binary_evol.test_BinaryStar
import posydon.tests.binary_evol.test_SingleStar
import posydon.tests.visualization.test_plot1D
import posydon.tests.visualization.test_plot2D
import posydon.tests.binary_evol.SN.test_profile_collapse

[ssbvr]

I also suggest enforcing a code formatting convention. Everyone can adopt this if they come in a .vs_code file. I think it's a good idea to trigger the formation when saving a file. When this is in place and POSYDON has been reformatted, add stylistic automatic checks with a GitHub action so that future code will comply with the coding convention.