Make it possible to run models in parallel

For the use case where we run two (or more) experiments with minor differences in one notebook, it would be really nice if they could execute in parallel. E.g.

```
discharge1
while reference_model.time < reference_model.end_time:
reference_model.update()
discharge = reference_model.get_value("discharge")
discharge1.append(discharge)

discharge2
while experiment_model.time < experiment_model.end_time:
experiment_model.update()
experiment_model.set_value("soil_moisture", .........)
discharge = experiment_model.get_value("discharge")
discharge2.append(discharge)
```

There are different ways to accomplish this; it would be nice if we could offer an easy way for the user using the ewatercycle interface, but this example shows it should at least be flexible enough to have custom statements in the second loop.