Store variables and constraints in separate dictionaries

[coroa]

ie. be less magical about whether one is handling a variable or a parameter.

Maybe provide getters var/con.

[coroa]

There are three possible styles:

1. Store them at the EnergyModels type:

   mutable struct EnergyModel <: AbstractModel
      components::Dict{Symbol,Component}
      subnetworks::Dict{Symbol,SubNetwork{EnergyModel}}
      buses::Dict{Symbol,Bus{EnergyModel}}
      variables
      constraints
      data::Data
      jump::Model
   end

   a) Use a flat dictionary below the EnergyModels object net.variables[(<class>, <var>)]. Let's call that PowerSimulations.jl-style; only that they keep a canonical model on which they do it and do not have the class complication, instead they have for instance :Pth for thermal dispatch, :Ph for hydro dispatch, :Pr for renewable).
   b) Use a stacked dictionary net.variables[<class>][<var>] and split the tuple; that's more like how PowerModels.jl does it.

2. Add the variables and constraints dictionary instead to the component types, like

   struct Generator <: OnePort
      model::EnergyModel
      class::Symbol
      variables
      constraints
   end

3. In the previous/current version, I used to store them on the JuMP model as: model.objDict[Symbol(class, :(::), var_name)], ie for example Symbol("onwind::p") unfortunately the separator :: is stupid to write. We might keep that additionally so that the JuMP model is self-contained.

[coroa]

Less magical about constraint variables is unfortunately not possible using the current format of JuMP extensions.

We're storing variables and constraints side-by-side in an objects dictionary on each component (so a variant of option 2 above), basically JuMP's model.obj_dict is made local on each component, using the thin ModelView wrapper.