getvars(::Formulation) should return a membersvector

[vitornesello]

To avoid the need of both filters that take a tuple and the own object.
Currently it returns a VarDict

[vitornesello]

Not trivial.
We could change:

function Base.filter(f::Function, vec::MembersVector)
    MembersVector(vec.elements, Base.filter(e -> f(vec.elements[e[1]]), vec.records))
end

to

function Base.filter(f::Function, vec::MembersVector)
    MembersVector(vec.elements, Base.filter(e -> f(Pair(e[1],vec.elements[e[1]])), vec.records))
end

where e[1] is the Id and vec.elements[e[1]] is the Variable itself.
But the thing get uglier and we will have to check if it allocates memory to do Pair(e[1],vec.elements[e[1]])

[guimarqu]

A way to solve this issue is to replace :

const VarDict = Dict{VarId,Variable}

by

struct VarDict
    variables:: Dict{VarId,Variable}
end

and do the same for constraints. What do you think about that @vitornesello ?

[vitornesello]

This seems good to me. I think it solves the problem