added initial join visitor
added DataFrameJoinVisitors
using VisitorSetEqual as a policy to generate the equal method in DataFrameVisitors and DataFrameJoinVisitors
policy generate the hash method
policy generate less and greater
generalize DataFrameVisitorsHasher
generalize DataFrameVisitorsEqual
generalize less and greater predicate
generalize DataFrameVisitorIndexMap and DataFrameVisitorIndexSet