The following program reads web page links from an input txt and builds a catalog
with the following features:

-All the different page IDs are stored in an AVL tree. IDs are positive integers

-The neighbours of each link in the AVL tree are organized using a doubly linked list which features
 insertion of a new link as well as deletion of an existing link.

-The neighbours are stored in increasing order

The program reads every command from a commands txt file which will include the
following commands (capital letters) : READ_DATA, END, INSERT_LINK, DELETE_LINK
and prints the complete list of all the links in the command prompt.

An example of the commands txt is given:

    READ_DATA
    INSERT_LINK 5 3
    INSERT_LINK 15 3
    INSERT_LINK 9 8
    INSERT_LINK 5 7
    DELETE_LINK 9 8
    INSERT_LINK 5 9
    DELETE_LINK 5 7
    INSERT_LINK 15 3
    INSERT_LINK 9 2
    INSERT_LINK 9 8
    INSERT_LINK 18 4
    END

READ_DATA exists only once in the beginning of the txt and the END once in the
end. If during INSERT_LINK the link already exists, then nothing is inserted. Also,
if the link that is commanded to be deleted using DELETE_LINK does not exist, then
nothing happens and the program continues.