store more useful information in symbolic struct

[baggepinnen]

The julia type is required to create a struct connector, like

@connector function StructInput(; structdef, name)
    n = Symbolics.juliatype(structdef)
    @variables u(t)::n [input = true]
    ODESystem(Equation[], t, [u], []; name = name)
end

where the symbolic variable u is of a struct type.