Allow a variable to be annotated anywhere it appears in a model

[MichaelClerx]

In CellML 2.0 it's no longer possible to see from the interface if a component 'defines' the variable.
In fact, the whole concept of ownership of variables has gone, which makes sense if you think of DAEs instead of ODEs (I'd have rather kept ODEs personally, but here we are).

So when CellML 2.0 comes along, we'll have to update code in various places so that you can annotate a variable regardless of its interface