Copy emission price trajectories to MESSAGEix-Transport scenarios

[khaeru]

This should allow to roughly simulate the effect of the policy workflows applied to the base model in e.g. the ScenarioMIP/SSP update projects, without the more extensive changes needed to either (a) make those workflows applicable directly to MESSAGEix-Transport or (b) build MESSAGEix-Transport on the scenarios resulting from those workflows. ((b) in particular is complicated by the fact that the workflows shift y₀ to 2030 (maybe via 2025); .model.transport currently only has data and code for parametrizing y₀=2020.)