Refactor and fix VLNS propose function

[Nicolae Filat]

The current implementation of the enumerate function looks like this.

function enumerate_neighbours_propose(enumeration_depth::Int64)
    return (solver::Solver, path::Vector{Int}, dict::Union{Nothing,Dict{String,Any}}) -> begin
        return BFSIterator(solver)
    end
end

This, however, has some mistakes:

1. The path is not taken into account at all by this function. That means that instead of enumerating with BFS at a specific location this just enumerates bfs from the root.
2. The dictionary contains that subset of rules that should be used. This is currently not implemented
3. Maybe refactor the code to now returns functions but use something else if possible