Enlarged board for outside board detection

[MarkZH]

Use a board representation that 12x12 to detect when moves are outside the board. The 8x8 actual board sits in the middle. When generating moves step-by-step, the border around the board allows for detection of moves that exit the board without needing if statements based on rank and file calculations. A two-square border is needed for keeping knight moves on the board. This should simplify the below code.

Genetic_Chess/src/Game/Square.cpp

Lines 82 to 94 in a637126

	Square& Square::operator+=(const Square_Difference& diff) noexcept
	{
	assert(std::abs(diff.file_change) < 8 && std::abs(diff.rank_change) < 8);
	const square_index_t new_rank_index = square_index + diff.rank_change;
	if(new_rank_index/8 != square_index/8) // make sure file did not change (happens if new square is off the board vertically)
	{
	square_index = invalid_index;
	return *this;
	}
	square_index = new_rank_index + 8*(diff.file_change);
	return *this;
	}

[MarkZH]

Another important improvement is that Square_Difference only requires a single int to specify the move (the square index difference). The linear algebra operations on Square_Difference (are_parallel(), same_direction()) will need some thought.

[MarkZH]

A 10x12 board with 4 extended ranks and 2 extended files is sufficient because a knight jumping horizontally off the board from the edge will always end on an index that is too large (either through wrapping a negative value off the a-file or landing on a too large index off the h-file.

Reference: https://www.chessprogramming.org/10x12_Board

[MarkZH]

Would a const std::bitset<120> be faster than index comparisons for out-of-board determinations?