improve TopArchiveList methods
improve TopArchiveList methods

- use TopArchiveList in the methods delcartion as the
  methods are specific to TopArchiveList
- return Inf for best_fitness() if the TopArchiveList is empty

JuliaPackageMirrors/BlackBoxOptim.jl,multidis/BlackBoxOptim.jl,tkelman/BlackBoxOptim.jl,JuliaPackageMirrors/BlackBoxOptim.jl,JuliaPackageMirrors/BlackBoxOptim.jl,tkelman/BlackBoxOptim.jl,tkelman/BlackBoxOptim.jl,multidis/BlackBoxOptim.jl,multidis/BlackBoxOptim.jl