Automatic port rates with hierarchical actors

[farresti]

Since the PiSDF semantic does not allow having different data rates on the date port of a hierachical actor and its inner data interfaces, it would be nice to have them automatically filled up.

For instance when creating a hierarchical actor from a .pi file, all the data ports should have their data rates already filled up with proper values since they can not differ from their inner definition in the subgraph.