XTCE13 — clarify semantics of context cals (possibly same as context alarms)

Clarify that the list of contexts is in a specific ordering, and that as it stands now the <xsd:sequence> is in an order and that's the ordering. Or, consider adding a required "ordering" attribute to enforce the order. Or perhaps that subtly changes the semantics and we don't want to do any of it in which case we might say that...