Do not explore entire graph for each clock cycle

[mortbopet]

Currently, the entire graph is explored for each clock cycle, to ensure correctness. However, this exploration in terms of properly sequencing the propagation operations is only required once.

This task should

1. save the result from the exploration into a stack of components which needs to be propagated
2. use this stack for all future clockings

Relates to: #9

[mortbopet]

Already done, the design creates a propagationstack, which is the sequence of ports to be updated.