Allow providing other indexers in `Dataset.xoak.sel()` 

It would be nice to be able to write

```python
>>> ds.xoak.sel(lat=..., lon=..., time=...)
```

even though `time` is not part of coordinates used in a xoak index.

It's a bit more convenient than writing

```python
>>> ds.xoak.sel(lat=..., lon=...).sel(time=...)
```