New internals to handle local assets

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:

```python
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](https://github.com/euroargodev/argopy/blob/cbc1139b2252b3798c0294d3aebf98ede97bc6e3/argopy/utils/accessories.py#L269) to easily access static files distributed with argopy:

```python
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.