Simplify time slicing in Comparer.sel()

The time slicing in `Comparer.sel()` uses an overly complex chain:

```python
d = d.sel(time=d.time.to_index().to_frame().loc[start:end].index)
```

This could likely be simplified to:

```python
d = d.sel(time=slice(start, end))
```

which is already how `raw_mod_data` is filtered two lines below.

See `src/modelskill/comparison/_comparison.py` around line 855.