end date should not be required

[crotwell]

Spec says end_date is required, but text says it should be omitted if currently active.

[speleos]

The proposal is aimed at describing an experiment after acquisition, for which end_date is important to have to sort out which data is available.

However, for ongoing experiments, metadata needs to be generated at the same time data is collected. It would make sense to leave the field optional and only require it upon archival.