Fix bug with wall on the last Board column.
Add edge filter to board graph.
Rename Board::pawn_node_ into pawn_nodes_.
Rearrange Board private methods.
Use typedef for vertices in BoardGraph.
Fix edges filtering in BoardGraph.
Add Board method to check if wall can be added.

- add method Board::try_add_wall to check if wall can be added;
- add utility method Board::side_nodes;
Add default ctor to BoardGraph class.

Initialize internal graph with additional method.
Add board size checking.
Add BoardGraph init parameters checking.