resource_family for the resource segmentation #106

Problem. res_types_vec(𝒫) segments the resources by concrete type, and the resource-specific functions (variables_flow_resource, constraints_resource, constraints_couple_resource, also their counterparts in EnergyModelsGeography) are called once per segment. An extension package with a family of resource types, say AbstractElectricPower with one concrete type per power-flow formulation, registers its JuMP variables (reactive power, voltage magnitude) inside these calls. A name can be registered once per model, so a case can contain a single concrete type of the family. Mixed cases, for instance a transmission resource with one formulation coupled to a distribution resource with another through a transformer node, are impossible without name mangling per type.

Proposal.

resource_family(p::Resource) = typeof(p)
res_types(𝒫)     = unique(map(resource_family, 𝒫))
res_types_vec(𝒫) = [Vector{rt}(filter(x -> resource_family(x) == rt, 𝒫)) for rt ∈ res_types(𝒫)]

The package sets resource_family(::AbstractElectricPower) = AbstractElectricPower, receives all its resources in one call, creates each variable once over the union and dispatches on the concrete types internally.

Why non-breaking. The default returns the concrete type, so the segments are exactly the ones of today. Only packages that define the trait see a difference, and only for their own resources. The three call sites in EnergyModelsBase and the four in EnergyModelsGeography do not change.

Tests. Two resource types with a common family return one segment typed with the family; types without the trait keep separate segments.