css-to-obfl: support a sequence of input documents

[bertfrees]

(Or alternatively, the input could be a concatenation of documents on which the CSS has already been applied before the merge; should come down to the same thing.)

The first consequence I can think of is that if a different @volume rule is applied to different documents, all except the first rule will be (silently) ignored. We currently can not handle this correctly because in OBFL volume-template is not part of layout-master (see braillespecs/obfl#34). Note that this is also the reason why we can't implement the volume property.

I'm not sure whether there are other consequences I didn't think of yet.