Versions of registers

Bane NOR has requested to be able to specify a version on designators: https://www.railml.org/forum/index.php?t=msg&th=938&start=0&

While major changes in the meaning of a rulebook entry should not occur without a changed rulebook register name, the topic of versions, revisions etc. of registers and external documents will become relevant at other situations, too. Therefore, a general approach on how the schema can deal with these revisions and versions of external resources (not only in infrastructure, but maybe also in rollingstock or timetable) may be useful for future railML versions.