[Dev] Solution

[ocots]

Add:

• tests on solution: build_solution
• getters
• plot extension
• save extension

[PierreMartinon]

Hi @ocots
Solution building seems to work (I added the include for solution.jl)
I noticed you renamed the objective as cost, any particular reason ?
I don't necessarily mind but we should keep the objective(sol) getter for backward compatibility.

Edit: for variable dependent problems I get an error with v (a Vector{Float64})

EXIT: Optimal Solution Found.
v [2.000031993268013] Vector{Float64}
ERROR: MethodError: no method matching CTModels.VariableModelSolution(::String, ::Vector{String}, ::Float64)

Closest candidates are:
  CTModels.VariableModelSolution(::String, ::Vector{String}, ::TS) where TS<:(AbstractVector{<:Real})
   @ CTModels ~/.julia/packages/CTModels/Y4fsV/src/types.jl:98

[ocots]

• I have renamed cost by objective.
• Ca devrait être bon pour la variable.