Better support for specifying state variables of composite machines

Following [this discussion](https://julialang.zulipchat.com/#narrow/stream/230248-catlab.2Ejl/topic/Internals.20of.20Discrete.20Machines.20via.20AlgebraicDynamics), it is clear that the current system places the onus of proper bookkeeping of the state of a composite machine on the user. A new feature that allows the user to use the wiring diagram to specify a state as a composite would reduce some of this burden.

There is also interest in having "stateful" machines, i.e. those which are equipped with a current state. This may have performance implications.