FIX: Adopt new Sax backend format

[jan-david-fischbach]

fix previous commit by manually converting to scoo.
Should probably be replaced by appropriate use of analyze_instances going forward.
@joamatab @flaport
should fix gdsfactory/gplugins#305

[flaport]

Thanks @jan-david-fischbach . No need to use analyze_instances in this case as we're working with raw s-matrices (not models producing s-matrices). Your solution is pretty good. Only extra change I made is to pin sax>=0.11.1.