New method getResourceRequiredAttributes

- add method getResourceRequiredAttributes from resource for group_resource and group attributes
- method has parameter "workWithGroupAttributes" 
- add simple test for this method
- fix of javadoc in AttributesManagerBl (PrivilegeException is not correct here)
Merge branch 'getResourceRequiredAttributes_for_group_resource_and_group' of https://github.com/stavamichal/perun into stavamichal-getResourceRequiredAttributes_for_group_resource_and_group

In order to fixup issues commented on GitHub.
CORE: small fixes to implementation of getResourceRequiredAttributes()

- Save up db query when checking if group and resource are from the same VO.
- Throw ConsistencyErrorException if method retrieves attributes from different
  namespace than group, group_resource.
- Trimmed whitespace