Combined my files in

Defined a comparison operator to sort the map. Made the relation class
friends with itself and the grammar class (means that any function
inside relation or grammar can access and change the private
data/functions of a relation). Removed unnecessary get functions in
relation. Made the destructor for the relation class. Implemented show
in relation. Finished insert_into and delete_from functions and tested
them. Tested cross product. I am also thinking that all the relational
algebra operations in the relation class will need to return a
*relation.