Define a type for ancillary services

[BSnelling]

Ancillary service offers are currently represented by a KeyedArray{Float64, 2}, where the keys are service provider ids x datetimes and the values are prices ($/pu). A missing in the keyed array means a provider did not submit an offer for that service at that time.

This data structure is insufficient to store all the information needed for other market clearing formulations. Defining a type would be useful because:

• more information about the offer can be stored
• different ISOs name the services differently but a lot of the concepts overlap so encoding characteristics about the ancillary service itself should facilitate code reuse in the models package

AncillaryService type structure

• offer curve::EnergyBids: as with other offers, ancillary service providers can submit bids as price-volume pairs with extra constraints e.g. multi-hour blocks (see #36 for EnergyBids details). Using the EnergyBids type should mean we no longer need to support missing offers because an offer of [(0.0, 0.0)] is equivalent
• name::Symbol: the name of the service
• generation_direction::Bool?: ancillary service offers can be offers for additional generation but also offers to reduce generation (e.g. regulation up and regulation down respectively). The field could be a Bool or singleton types Up <: GenerationDirection, Down <: GenerationDirection
• online_offline::Bool: ancillary service requirements can specify requirements from both online resources and offline resources