added check_columns_partially

check_columns_partially looks through the column positions of
a board_matrix for almost n_in_a_rows (n spaces lined up
where n-1 of them have been played on by the same player, and
the last one is empty). A situation where the player in question
could win on their next turn. It then note those spaces down
on the n_in_a_row_position_matrix and returns a reference to the
n_in_a_row_position_matrix.
check_rows_partially: made code clearer

explicitly defined some more things into variables