[Rule] Station StartDate and Channel Start Date must not be null

[timronan]

The stationxml-validator should require Station and Channel StartDates. Stations and channels cannot easily be associated without a StartDate. The association logic assumes that a null timestamp is assigned as a negative infinite time (null StartDate) or an infinite time (Null EndDate). Following this logic, a station or channel can't have a null StartDate because stations and channels have not been operating for all of existence.