MBC interface spec

[luke-kiernan]

When it comes to MBC things, here's the interface I'm thinking will work (based on @GabrielKS very helpful diagrams). We define 3 functions here in IOM:

1. validate_occ_component. This is extended by downstream packages for device-specific validations: e.g. renewables shouldn't have startup costs.
2. process_market_bid_parameters! and process_import_export_parameters!. This adds the correct parameters to the various containers, based on the formulation.
3. add_pwl_term!. This is where we add the relevant terms to the objective function.

TBD: updating objective function terms.