Start-up fuel consumption

To add to the various start-up issues; this would add fixed amount of consumption during the start-up phase of an asset. It probably makes most sense for Links/Processes, since you can then set a non-zero withdrawal from bus0 for zero supply at bus{N>0}. This could be represented internally as a piecewise consumption curve with non-zero intercept.

I can see the value of also defining it for Generators (for global CO2 constraint setting). It would involve setting a fixed value during start-up of carrier consumption which is then added to the p / efficiency term when setting the global constraint.

Start by tracing how startup behavior is represented for Links/Processes, then compare the Generator path for carrier consumption and global CO2 constraints. Clarify whether both asset types are in scope, how the piecewise curve or fixed startup value should be represented, and identify tests for zero-supply startup and global-constraint accounting.