Parameter substitution should also act on leads
Builder parameter substitution currently only acts on the main builder. Any leads that are attached to the input system are simply ignored. The output system has always no leads.
When some observable of a finalized system is computed, the parameters are only specified once for both the central region and any leads. Therefore, it seems to me that it would be more consistent to make parameter substitution work in an equivalent way.
Do you see any reasons not to do this? (cc @jbweston, @anton-akhmerov)