CORE, RPC: support for getRequiredAttributes(sess, facility, list<service>)

CORE:
- adedd getRequiredAttributes method for facility and list of services
   (implementatiton should be fixed in future to use single select to DB
   instead of calling method for each service)
 - added checkAttributeValue for Group to api layer to fix javadoc references
 - fixed many typos in AttributesManager javadoc
RPC:
 - added getRequiredAttributes method for Facility and list of services
 - fixed up RPC javadoc