Add the possibility of adding combinations between stations

[BrunoSalerno]

• combination should be a relationship between 2 stations (table with station a and station b)
• it should be drawn whenever both stations are operative (unless we add a operative_from and operative_to fields, I'm not sure about that)
• If a combination involves more than 2 stations, it has to be drawn by pairs
• They can be added from a list that displays close stations