NML defines a Lifetime object, which can be related to a Network Object. This relation is currently unnamed.

Proposal: use the name "existsDuring" relation to identify this relation.

(I'm not sure if we need to name it in the syntax, but at least it is useful so we can refer to it in the document)

Meaning of the relation is as follows:
* A LifeTime object only has a single start and endtime.
* A Network Object can be related to zero, one or more LifeTime objects.
* If a Network Object is related to one or more LifeTime objects, this should be interpreted that the Network Object is 
guaranteed to be "in service" during these times. It implicitly can be assumed that it is not in service during non-
specified times.
* If a Network Object is related to zero LifeTime objects, there is no guarantee when the Network Object is "in service"
, but it may implicitly be assumed that it is in service from the moment of reception of the NML description till the 
publication of an updated NML description (whenever that may be).