Use argopy for ftp download of data

Instead of using the ftp library, one could use argopy to retrieve the multi-profile file of a float like this:

```python
from argopy import DataFetcher as ArgoDataFetcher

wmo = 7900549
ds = ArgoDataFetcher(src='gdac', mode='expert', cache=True).float(wmo).to_xarray()
ds = ds.argo.point2profile()
```

In the notebook example, I think we could follow on to work with this dataset, but one could simply save it as a netcdf file as well:
```python
ds.to_netcdf(wmo+ '_prof.nc')
```