For each dependecy in a simulation graph, create a map containing:
 - key: name of the custom component.
 - value: a map with:
   - key: each InputPortNumber
   - value: a list of OutputPortNumber combinatorially connected to the
            input.
 An input is considered combinatorially connected to an output if there is at
 least one logic path connecting an input directly with the output. In other
 words, there must be at least one route from the input to output that does
 not encounter any synchronous component.
 Return None if such information cannot be inferred, for example if there is
 a circular dependency.

Tells wether a component is clocked or not. Note that Custom components may be clocked (cannot tell without recursively analysing them), so they are considered synchronous.

Given a map of combinatorial routes from inputs to outputs for every simulation graph, perform a lookup to find the combinatorial routes from a given input to the outputs. Then filter the outputs of the custom node to only point to the combinatorial children (i.e. the ones connected to the combinatorial outptus).

used to do asynchronous cycle checking on atomic components with non-trivial asynch paths. should this, or something like it, also be used for in dependency cycle checking? returns Some (async outputPortNumbers from inputPortNumber) if component is hybrid, otherwise None.

Find out whether a simulation graph has some synchronous components.