What is the list of possible Argo parameters for the DataFetcher params argument ?

@delphinedobler suggests that it would be nice to have a facilitated (machine-to-machine, non-experts) access to the possible list of Argo parameters for [the BGC DataFetcher ``params`` argument](https://argopy.readthedocs.io/en/latest/user-guide/fetching-argo-data/data_set.html#the-params-argument)

This is poorly document in the API auto-generated pages (it's [not listed here for instance](https://argopy.readthedocs.io/en/latest/generated/argopy.fetchers.ArgoDataFetcher.html#argopy.fetchers.ArgoDataFetcher))

It could also be made available for all possible values of the ``ds`` argument, ie the choice of the dataset, ``phy`` or ``bgc`` at this point.

This new feature could go like this:

```python
from argopy import DataFetcher

fetcher = DataFetcher(ds='bgc')
# or:
fetcher = DataFetcher(ds='phy')

# or with a selection point:
fetcher = DataFetcher(ds='bgc').region(box)
fetcher = DataFetcher(ds='phy').region(box)

# New API:
fetcher.params_available
fetcher.params_description

```
The new fetcher attribute ``params_available`` could return a list of strings like: ``['DOXY', 'BBP700']``.

While the complementary (for Argo data beginners) ``params_description`` could return a dictionary with available parameters as keys and some meta-data description as values, probably retrieved using the NVS.