Prevent obs from having both value and members

Obs group parents should not, themselves, have a value (see this thread). Until obs groups can be separated from obs (TRUNK-1858), we should at least prevent grouping obs (parent obs with members) from having a value.

Dev notes

In the ObsValidator

• Throw an APIException if an obs with members has any value filled in

In the Obs.java class:

• In the Obs.addMember method, check to make sure the current obs doesn't have any values. Throw an APIException if it does.