Main network validation

[SouthEndMusic]

What In addition to https://github.com/Deltares/Ribasim/issues/710, new validation rules arise from implementing support for the main network in allocation:

• [ ] when a subnetwork is connected to the main network with multiple edges, these edges should all end in a different node of the subnetwork.
• [ ] (Not specific to the main network): the allocation_network_id specified in the Edge table to denote a source should also exist in the Node table (and should indeed be an edge that connects the main network to a subnetwork).

[ghost]

This will come back once #2255 is completed.