make deadline a iao:conditional specification

[StroemPhi]

So far we have modeled the various deadlines associated with an academic event (e.g. submission deadline) as data properties of the academic event class. As we will need to be able to say more about deadlines in the future, it seems better to model them as sublcasses of iao:conditional specification.

The rationale behind this is:

• a deadline is set by the organizers and is thus part of their plan that gets realized in the event
• a deadline is a conditional specification, because it specifies a date until which a participant of the event has to do something

[StroemPhi]

While making the needed deadlines subclasses of iao:conditional specification, it also made sens to define call for papers and call for abstracts as subclasses of iao:action specification as those are always the specifications in which the deadlines are defined.

[StroemPhi]

For the literal representation the data property has datetime value was added under iao'has representation to pürovide a value for the dealine with xsd:dateTimeStamp restriction.