Folder structure for generating subset results via external model

Currently for each level of the subset simulation a different folder with a timestamp is created, this can result in confusing folder structures.

We suggest to implement something like this, maybe also add the method that was used to the other reliability estimation methods.

```
2025-29-08-12-00-subset
  |-------- level-1
      |------ sample-001
      |------ sample-002
  |-------- level-2
```