Implement new generalised type for storing energy bids

[BSnelling]

Call this new type EnergyBids for the sake of the issue. EnergyBids would replace the KeyedArray{Vector{Tuple{Float64, Float64}}, 2} that currently represents a set of bids in a System. The keys of these KeyedArrays are bid ids x datetimes and the values are vectors of price-volume pairs ($, pu). Initially this was sufficient to represent the concept of energy bids. In order to support the formulations of other ISOs there are additional requirements for the EnergyBids type:

• price sensitive bids can still be stored as KeyedArray{Vector{Tuple{Float64, Float64}}, 2} where the axis keys are bid ids x datetimes
• multi hour bids - bids submitted with a true multi hour flag have to clear as a block (either they all clear or none do), this can be stored in the EnergyBids as a KeyedArray{Int} where the integers act as an id for a block of multi-hour bids
• fixed bids - bids that are "fixed" must clear at the specified volume or not at all. The alternative is variable type bids which can clear at a volume up to the specified volume. This can be stored in EnergyBids as a KeyedArray{Bool}

So the type would look like:

struct EnergyBids
    """
    Multi-hour block id. In a multi-hour block either all bids clear or none do.
    """
    multi_hour::KeyedArray{Int64, 2}
    """
    Bool indicating whether the bid is a fixed type which can only clear at the given price
    and volume.
    """
    is_fixed_type::KeyedArray{Bool, 2}
    "Bid curve, `KeyedArray` where the axis keys are `generator names x datetimes`."
    bid_curve::KeyedArray{PriceSensitiveBid, 2}
end

Additional requirements

• it may be useful to have a way to differentiate between supply and demand EnergyBids, either by adding a field to EnergyBids or using subtypes