Update variable addresses provided by external models
This method sets attributes including parent_model,
parent_instance, uid, a, n, e_code and
v_code. It initializes the e and v to zero.
Instance of the parent model
link_external does not check if the ExtVar type is the same
as the original variable to reduce performance overhead.
It will be a silent error (a dimension too small error from dae.build_pattern)
if a model uses ExtAlgeb to access a State, or vice versa.