New internals to handle local assets

[gmaze]

This needs appeared in work for #535 and #542

In the current master branch there's a path 2 static asset files available in the utils sub-module:

from argopy.utils import path2assets

But this is not satisfactory, since a method will be responsible for navigating the static asset content.

that's why in #545 we introduce a new Asset utility to easily access static files distributed with argopy:

from argopy.utils import Asset
Asset.load('data_types')
Asset.load('schema:argo.float.schema')

but still, this does not support assets that would need to be downloaded/accessed after argopy is installed by users and located elsewhere than the installation root path.