Fares belong to multiple networks

### Describe the problem

I am trying to model London transport fares. However, for the purpose of fares, different fare products have different interpretation of the network. For example, using an Underground ticket, the network contains only Underground and DLR routes. Using a National Rail ticket, the network contains only National Rail routes. Using a contactless payment card, the network contains both National Rail and Underground, DLR routes where they can be joined together as a single fare leg.

### Use cases

To model different fares used for different, overlapping network within a region.

### Proposed solution

relax the requirement in `route_networks.txt` to allow a route to be assigned in multiple networks.

### Additional information

_No response_