Add tests for API to add Flows between existing Nodes

[p-snft]

There is an option to add flows between existing nodes in an energy system. However, there was no test, so it could not be considered as part of the API. This PR adds tests to assure the option is there.

node1 = Node()
node2 = Node()
energy_system.add(node1, node2)
node2.inputs[node1] = Flow()
node1.inputs.update({node2: Flow()})

[pep8speaks]

Hello @p-snft! Thanks for updating this PR. We checked the lines you've touched for PEP 8 issues, and found:

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2023-10-10 08:58:11 UTC

[p-snft]

I learned from @gnn (on another channel) that it was already possible to add Flows:

node1 = Node()
node2 = Node()
node2.inputs[node1] = Flow()
node1.inputs.update({node2: Flow()})

I think, I will rewrite my tests for that API and document it properly.

[p-snft]

As this is just adding tests and I got feedback on other channels, I will probably merge this after adding documentation.