Better Support for Self-closing (empty) fields

[aminya]

Now to define a self-closing field you should define a separate type. It would be nice if we can just specify its self-closing in the parent type definition.

 @aml mutable struct Scoreinstrument "score-instrument"
    name::String, "instrument-name"
    ensemble::UN{Int64} = nothing, sc"ensemble", positive
    solo::UN{Int64} = nothing, sc"solo", positive
end