Improve generator stub scaling

### Problem

Right now, only branches connecting renewable generators are scaled, and the identification process for stub branches is rudimentary: we only trace back into the network until it 'branches' (we encounter a bus 3+ branches connected to it), but that doesn't identify branches where there is a 'Y' downstream of a single connection to the mesh network. I.e., we're not accurately identifying all 'bridges' https://en.wikipedia.org/wiki/Bridge_(graph_theory). Since the Seams study will require a decent quantity of new gas capacity, and since there are often several units at one gas plant behind a single point-of-interconnection to the main grid, we want to fix these issues.

### Proposed solutions

- Refactor `scale_renewable_stubs()` to allow an arbitrary set of resources to upgrade
- Refactor `scale_renewable_stubs()` to better identify bridge branches, using the `networkx` package or similar (see https://networkx.github.io/documentation/stable/reference/algorithms/generated/networkx.algorithms.bridges.bridges.html)