A new method to export an ArgoIndex selection to a DataFetcher ?

This new method to be implemented in the ArgoIndex would make it possible to convert the index to a DataFetcher object to load the actual data.

This could go like this:
```python
from argopy import ArgoIndex

idx = ArgoIndex().query.box([-60, -55, 40., 45., '2007-08-01', '2007-09-01'])

fet = idx.to_datafetcher(src='erddap', mode='research')

ds = fet.to_xarray()

```

The ``ArgoIndex.to_datafetcher()`` would know how to map search queries to the available [data selection point of a DataFetcher](https://argopy.readthedocs.io/en/latest/user-guide/fetching-argo-data/data_selection.html).