Block relationships: add notion of block dependencies

[jbohren]

Add the option of designating some blocks as dependent on others. Allow for cyclic dependencies, i.e. if two blocks are dependent on each-other, they must both either be active or inactive at any given point.

[konradb3]

I was thinking about something similar. I would no like to define dependencies explicitly. Instead we can annotate the input port that they require data (>= 1 active writer) similarly to exclusivity (<= 1 active writer). I think that those two mechanism can solve all possible dependencies.