build_future_hwys()

The way it generates a future network is to update the base year to another year. When that happens, both the highway links and the project table will be affected. Say you want to update to 2020's highway network. Then of course, if you apply another year of changes to it, then you will have 2021's highway network! The first idea behind this method is that every year's highway network builds off of the previous year's highway network.

The second idea is to think of project changes arithmetically.

TODO: add the node (arterial, ramp, ghost) checks.