Add 'length overrides' to places where transmission distances are assumed via haversine

Currently, we assume that all transmission lines follow minimum great circle distances point-to-point. We do this for:
- calculating costs of branch upgrades
- calculating impedances for new branches
- calculating MW-miles of upgrades

It would be nice if we could pass 'overrides' on a per-branch basis that would substitute for great-circle distances, where we have better estimates of real distances.