[FIX] Explicitly check that local_nb_nodes.json is an existing file

[alyssadai]

• Closes API (sometimes) exits when local_nb_nodes.json is a directory #81

Changes proposed in this pull request:

• Instead of simply checking if the path exists (which can cause problems if the path points to a directory instead of a file), check if local_nb_nodes.json is an existing file

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• Tests have been added

For bug fixes:

• There is at least one test that would fail under the original bug conditions.

[rmanaem]

🧑‍🍳

[surchs]

🚀 PR was released in v0.2.1 🚀