made toString according to the pdf

In the pdf it says that the toString metods should reflect the enconding
described for the battlefield class
Changed atributes to protected

I think its ok to do this but i'm not sure
Correct typing error
Started working in getBattlefield()

Also add an execption to the void setBattleFieldElement. Maybe the
problem with the super in the constructers that was discussed in the
meeting can be solved with something like this