Unify usage of validity across Common and RTM

ATM the class organizationalUnit is declaring a child isValid based on the type CalendarTimePeriod in Generic, while in RTM the NetworkResource and some other classes declare a child element isValid using the RTM type RTM_Validity. The difference between both is, the fact that RTM_validity is using the type xs:date, while CalendarTimePeriod is using xs:dateTime. It was mentioned that this should be unified. One of the benefits would be an easier documentation and less redundant types in the schema.