Thanks for the refactor, according to the latest demo in EasternCapacityScaling.ipynb, the overhead of using the framework is largely reduced, which I think is good enough for now. A few questions I would like to discuss at this point:

1. Are we using capacity_planning_demo.ipynb any more? If not, should we remove it from the repo.
2. This PR is to reduce the user's overhead so that the steps of generating the target capacities for next scenario are shortened and clean. Should we address the package/folder restructure issue in this PR as well, i.e. create a design folder, put design_transmission and this framework into it and appropriately rename the folder, such as scaling, etc. @rouille
3. How about the test folder restructure, i.e. put all data files into a data folder.

I'd save moving files around for a followup PR... Making code changes and moving files in a single PR just makes things more confusing, as we learned with the Eastern Demand PRs.

After this PR gets in, folder restructuring, we might think about incorporating direct change table output...

After this PR gets in, folder restructuring, we might think about incorporating direct change table output...

Sure. Let discuss this probably in the follow up PR. I'll run an integration test via the demo notebook this afternoon to check whether the outputs match previous one. If it looks good, I'll approve this PR.