Zebra: Support cyclic graphs #81
Description
The input of a Block in a Zebra must support linking to an output of the Zebra.
As the Blocks of a Zebra update their outputs 20ns after their inputs change, this may be possible to resolve without having a cyclic dependency that gets stuck by marking blocks are dirty once they have been traversed once; or caching all outputs of a Zebra, such that they are available from the External component of the System simulation?
