circulations should be optional

Andreas Tanner requested optional circulations: http://www.railml.org/forum/ro//index.php?group=2&thread=100&id=359

We use blocks to describe vehicle duties within a day, and circulations to concatenate them within a multiple day schedule. Now we have a case where there are no circulations present (as they are defined only in a later planning stage). I think this should be valid, and therefore I propose to relax the standard in that point.

Variante A: circulation optional Variante B: insert with minOccurs=0 into the sequence of the element