Add nested controller capability : enable analysis through layers of architecture

[rreis]

Nancy Leveson decribes a layered approach, where a high level architecture is progressively detailed. This is quite a useful approach as it helps support architectural trade-offs across all layers, with adequate level of detail to each.

The same mechanism used in capella for the Data Flow Blank, where functions can be decomposed, would suffice for this.

For instance... bellow some prototyping using DFB to simulate modelling of STPA controllers. This was achieved by cloning the diagram with the functional decomposition and then removing the internal functions through the "Functions" option.

The high level concept diagram can be further simplified by the user using "categories":