Include in argopy as inner function

Hi folks,
Great method you developed here !
Do you mind if we integrate this into the [argopy](https://github.com/euroargodev/argopy) software ?

The new API could look like this:
```python
from argopy import DataFetcher as ArgoDataFetcher
ds = ArgoDataFetcher(mode='expert').float(2900114).load().data
ds = ds.argo.traj_interp_terrain_following()
```
and the data set would have new LAT_INTERP/LON_INTERP variables with interpolated values

Getting topographic data would be made by argopy as well, so thta users wouldn't have to manage this

Since you work with panda dataframe this could be made quite easily I guess