Improve financial modeling in GenericPlant

[elenya-grant]

GenericPlant is intended to be used primarily for physics-based simulations and the financials were not a primary concern. Currently there are no tests or extended functionality to ensure GenericMultiSystem gives similar financial results as if systems were represented as other PowerSource objects. In summary, to ensure GenericPlant can also be used for financial results, the following needs to be done:

• Add tests for financial modeling in GenericPlant
• Add functionality so financial models can be specified per subsystem if using GenericMultiSystem