Mobile concentration plot not rendering in Permeation barrier modelling

In https://festim-workshop.readthedocs.io/en/latest/content/applications/task04.html#model-with-barrier
Error:
`/tmp/ipykernel_3252/2865152383.py:23: UserWarning: No artists with labels found to put in legend. Note that artists whose label start with an underscore are ignored when legend() is called with no argument.
axs[0].legend(fontsize=12, reverse=True)`

It is caused by the fact that `profile.times` is empty so nothing is plotted
-> fix by iterating over `profile.t` instead. Still `profile.times` should be populated since timestamps are specified when doing the export

```python
profile_exports = [
[F.Profile1DExport](field=spe, subdomain=vol, times=[100, 17000, 8e5])
for spe in model_barrier.species
for vol in model_barrier.volume_subdomains
]
```