fix: cache nxcg graph instead of coo representation

[aMahanna]

Proposing to remove the following cached objects:

• src_indices
• dst_indices
• edge_indices
• vertex_ids_to_indices
• edge_values

In favour of just caching the nxcg.Graph object that we create via the from_coo method.

Eventually we'll need to revisit this cache concept, because it immediately becomes out of date if a user modifies the NetworkX-ArangoDB Graph. For now, we can assume that the user will rely on G.clear_nxcg_cache to indicate that a new COO pull is required

[hkernbach]

LGMT, minor comments left.