Method key3

* Build a symetric base 3 key. Two symetric positions will have the same key. * * This key is a base 3 representation of the sequence of played moves column per column, * from bottom to top. The 3 digits are top_of_colum(0), current_player(1), opponent(2). * * example: game "45" where player one played colum 4, then player two played column 5 * has a representation in base 3 digits : 0