insert_operation using signal as reference

Currently, the id provided to insert_operation must be an operation. If it is allowed to be a signal, it will be easier to handle nodes with branches.

added Completeness Enhancement Signal flow graph labels

Alternatively, one could specify an operator + any {input or output} to place the scaling before/after?

In any case, since insert_operation seems to changes all operaton numbering, it is hopeless to use several insert_operation after each other.

It should be great if user could manually specify a unique ID or something to each operation, and then trust this is not changed. If the ID is a string, cascading can just pad a string in front of each ID, which identifies which SFG the operator came from.

Yes, the non-mutability (= recreate the SFG upon changes) creates quite a bit of problems, see #97 (closed)

changed milestone to %Autumn version