Keep graph_id if possible

Description of issue:

It would often be easier if a given graph id is kept while modifying the SFG. Now, this is the case when removing operations, but only sometimes when adding. The solution is to scan the SFG twice: once to determine which graph_id:s that exists (and which are duplicates), and once that assigns the operations/signals without graph_id:s or with a duplicated graph_id.