Subcomposition Iteration

Allow subcompositions to be itterated. Input and Output ports can generate lists of the iterations. If for example a subcomposition takes X, and outputs Y, the subcomposition iterator would create an input port List of X’s, and output port List of Y’s. An iteration number port could be added to current subcomposition node.

The feature request Allow changing single-value ports to accept lists (turn most nodes into iterators) also applies to subcompositions, so I’m marking this feature request as merged with that one.