Child molecules should have access to parent variables without using $parent

[madx]

Currently a child molecule has to use the $parent notation to access variables defined in the parent scope. It would be more natural to have a direct access to parent variables, with the ability of shadowing a parent variable by declaring it in a child scope. Once shadowed, a parent variable would remain accessible through the $parent notation. This could probably be implemented by using a proxy for the data variable in createMolecule