Make DIRECTORY Re-usable

The changes involved are as follows: - remove second invariant from DIRECTORY class (this is not appropriate anyway since it is never preferable for a class to know what it is being used by. - rename FOLDER.compositions to FOLDER.items, which would be more correct since the type of this attribute is already OJBECT_REF, meaning it can be any kind of item. - change 2nd invariant of FOLDER to allow any descendant of VERSION<T> to be an item, not just VERSIONED_COMPOSITION - move DIRECTORY and FOLDER into a small package of its own called "directory" in the Common IM - change the diagram in the EHR IM to show DIRECTORY and FOLDER being re-used this new package - change the demographic IM to show that DIRECTORY/FOLDER can be used there as well.