Modified diagonal buckets to use coordinates as keys rather than SAN strings. Changed _san_to_x_diagonal to _coordinates_to_x_diagonal and modified both to work on and return coordinates rather than SAN strings.
Reworked tests for _coordinates_to_(x)_diagonal to reflect change in game class functionality.
Added system for diagonal dictionaries with 64 keys each, representing every space on the board.
Added skeleton tests for _generate_diagonals functions.
Tweaked _coordinates_to_backward_diagonal. Test for it is now passing.
Added docstrings to skeleton tests.