Cost Model Refactoring

[ccoffrin]

Explore if adding support for polynomial costs into objective_variable_pg_cost (and similar functions) can be used to simplify the implementation of the objective functions.

Adding special cases in the PWL logic in objective_variable_pg_cost for the constant and linear functions (without the introduction of lambda variables) can also improve performance in some cases.

[ccoffrin]

This refactoring should also support mixing the types of cost models per component. Some generator should be able to be polynomial while others are pwl. Mixing should also work across generator and dcline components.