Allow the board Size to be dynamic

Hi
The board size was set at 4*4 and was not worked with any value.

I made some changes to allow the allow dynamic board size.

The update is simple but important . I have added an instance variable "boardSize" so we can set it with any value instead of 4 in all methods .
 I have changed all methods and added some codes to allow to set any value.

Also, I tested all methods to check thier work after changes. In addition, I changed Test class to suitable with changes in Board class.

Kind Regards,

Mansoor