Reorganize fuel price input data structure for clarity

[jrissman]

Take in from input variables and sum together:

• BAU fuel price before taxes
• BAU carbon tax per unit fuel
• BAU other taxes per unit fuel
• BAU subsidy per unit fuel

This edit is intended to clean up the "Fuels" page in Vensim and avoid errors when users supply data for the current variable fuels/BFCpUEbS "BAU fuel cost per unit energy by sector," which includes all taxes and subsidies inside it. (Meanwhile, the EPS requires the share of fuel price that is tax, and BAU subsidies, to be provided separately, and it can't distinguish BAU carbon taxes from BAU other taxes.) Taking in each of the four fuel price components separately on a per-unit-fuel basis could make the variable names and calculation flow cleaner and less confusing.

This edit should have no effect on the model outputs or calculations. It is entirely to make the model easier to understand and to adapt to new regions.

[jrissman]

Fixed in commit 11bc7b5 (as part of the work on issue #85).