obs_names and var_names added to fields

[FrancescaDr]

New feature: obs_names and var_names from AnnData can now be stored in PyG data by adding the variable to fields.

For example:

fields={
            "x": ["X"],
            "obs_names": ["obs_names"],
            "var_names": ["var_names"],
            "edge_index": ["uns/edge_index"],
            "y": ["obs/cell_type"],
        }

Note: additional update in basic.py because of numpy version 1.2.0 deprecated np.float