QoL: simplify `Settings.transient` and `Settings.final_time`

https://github.com/festim-dev/FESTIM/blob/31864c86690750ab854942c82110c3f711187246/festim/settings.py#L63-L65

It can be annoying to have to set both the arguments `final_time` and `transient` as it is kind of redundant.
If we provide a final time, then obviously the simulation is transient.

We could simplify this by removing the `transient` argument altogether and replace it by a property attribute of `Settings`.

```python
@property
def transient(self):
return self.final_time is not None
```