Make sequences more fine-grained

[xatapult]

I would advocate for making the definition of sequences more fine-grained. Currently you can only say that some port is either a sequence (0 or more) or not (always 1 document).

I often have situations where I have ports that emit 0 or 1 document. Or always more than one.

So at least the "DTD" distinction would be nice (?, *, +).

We maybe could add this to the sequence attribute of port definitions? Something like sequence="?"