refactor `QuantumNetwork` to support general quantum networks

[Canoming]

Checklist:

• [ ] Reviewers confirm new code works as expected.
• [x] Tests are passing.
• [x] Coverage does not decrease.
• [x] Documentation is updated.

I've refactored the QuantumNetworks to support general quantum networks and make it more compatible with usual tensor contraction methods.

To make things clear, I made a class diagram for the future development of the module