create a data class for a pair of vectors
create a registerer for VectorPairData
create a retriever for VectorPairData
[REFACTORING] rename VectorPairRegisterer into VectorPairDataRegisterer
implement functions to retrieve dead/generated methods in the specified revision
add fields for the retriever and the registerer for VectorPairData
create classes for settings of VectorLinker
create a class for threads to detect pairs of vectors
implement the main function of VectorLinker
[BUG_FIX] correct a worng spelling
[BUG_FIX] a bug was fixed that throws NullPointerException in the process for the last element of target revisions
[MINOR_CHANGE] a content for printing current states was modified