New method to interpolate dataset on theta surface

Some analysis require to have salinity values on iso-thermal surfaces.
It would be awesome to have an easy method in argopy to do that.
The API may simply look like this:

```python
from argopy import DataFetcher

ds = DataFetcher().float(6901035).data

dsi = ds.argo.interp_on_theta(theta=np.arange(0,40))
dsi = ds.argo.interp_on_theta(theta=np.arange(0,40), method='linear')
dsi = ds.argo.interp_on_theta(theta=np.arange(0,40), method='linear', fillvalue=np.NaN)
```

it is possible that such function already exists in the [python OWC software](https://github.com/euroargodev/argodmqc_owc)