Refactor ProductionCostExpression

[m-bossart]

Refactor the ProductionCostExpression in PSI which currently includes all of the costs aggregated into a single expression. The goal is to create new expressions which represent the constituent types of costs (e.g. FuelCost, VOMCost, StartupCost, ShutdownCost)

[ChenHaotianC]

Hi @m-bossart , I opened a draft PR for this issue: #1552. #1552

Summary:

• Added cost breakdown expressions for thermal generators: VariableCostExpression, StartUpCostExpression, ShutDownCostExpression, ProportionalCostExpression.
• Updated the thermal construct_device! path and shared helper(s) under devices/common/objective_function.
• Added tests in test/test_device_thermal_generation_constructors.jl to (1) check the sum of the components equals ProductionCostExpression, and (2) validate StartUpCostExpression against the delta in ProductionCostExpression in a forced-startup scenario.

Could you please take a look when you have time and let me know:

1. whether the chosen breakdown and where the terms are recorded looks correct, and
2. whether it is necessary broader test coverage beyond c_linear_cost_test or extension to non-thermal devices?

[ChenHaotianC]

For renewable devices, the current objective function construction only uses add_variable_cost! and does not include curtailment cost. Should curtailment cost be included in the objective function as well?

[ChenHaotianC]

I opened a new PR for this issue
#1563