null in startTime

[crotwell]

Do we really want to allow startTime to be null?

I worry that this will be a problem as startTimes are often used as part of the identifier for stations and channels, and to some extent networks.

[CTrabant]

Agreed, we should make it required.

[crotwell]

Here is a pull request that does this:
#8

Question, can end in fdsn-timeSpan be null? My feeling is no, but not sure if that is used for availability that is up to date with "now"?