Changes

Changes the structure of social networks and complete javadoc
Change the hashmap of the social networks by three independent strings.
To do this, the functions to store, modify and eliminate are eliminated
and the three strings with the getters and setters are added.
Adds a contructor with all the data of the entity.
Completes the javadoc of the entity and modifies some error message of
some function to correctly specify the error caused.