Promote index store to frontend feature

[gmaze]

The :class:IndexFetcher is a user-friendly fetcher built on top of our internal Argo index file store.

But for one familiar with Argo index files and/or who cares about performances, it may be interesting to use directly the Argo index store. We thus try to promote this internal feature as a frontend class :class:ArgoIndex.

As explained in the doc, we have to choose which store to use for index:

from argopy.stores import indexstore_pd as indexstore  # Rely on Pandas
# or:
from argopy.stores import indexstore_pa as indexstore  # Rely on Pyarrow

The goal here, is to have a new class ``ArgoIndex```:

from argopy import ArgoIndex

that will be indexstore_pa if Pyarrow is available or fall back on indexstore_pd otherwise.

[gmaze]

This could be as easy as to have some thing like this:

import importlib
if importlib.util.find_spec("pyarrow") is not None:
    from .argo_index_pa import indexstore_pyarrow as indexstore
else:
    from .argo_index_pd import indexstore_pandas as indexstore

class ArgoIndex(indexstore):
    pass

in a new argopy.stores.argo_index.py file
In this case, ArgoIndex will not be an alias and all the docstring will be easily inherited and available in the doc