Add support for setting variables using dotted paths

[madx]

Dotted paths can currently be used only to retrieve a variable's value. The user should be able to also do things like

set("path.to.a.variable", value)

[madx]

NO, we shouldn't do this, as we should not encourage using deeply nested items.

We can already update a variable easily, to update a field in an object I believe it's better to have some separate way for updating the object, then use set to update the whole object.