add retriever without retrieving any volatile information
modify the structure of IRetriever
add retrievers for each data class that retrieve volatile information, and implement the one for clone genealogies
implement retriever for clone class mapping
[BUG_FIX] make sure the retrieved elements are stored by the manager
implement retriever for clone class
implement retriever for code fragment mapping
implement retriever for code fragment
implement retriever for revision
implement retriever for segment
implement retriever for source file
implement retriever for version