GroupImp#getMembers and #getDeclaredMembers should return RangeIterator

for those cases where the total amount of members can easily be detected/calculated the
implementations of Group#getMembers() and #getDeclaredMembersOf() should return a RangeIterator.

so far i found that Group#declaredMembers() can be easily adjusted for those cases where
the group members are stored in a multivalued property.