function stdev( out, arr, mx, my, bool ) {
	var delta;
	var M2x;
	var M2y;
	var N;
	var i;

	M2x = 0.0;
	M2y = 0.0;

	N = 0;
	for ( i = 0; i < arr.length; i++ ) {
		N += 1;
		delta = arr[i][0] - mx;
		M2x += delta * delta;
		delta = arr[i][1] - my;
		M2y += delta * delta;
	}
	if ( bool ) {
		out[ 0 ] = sqrt( M2x / N );
		out[ 1 ] = sqrt( M2y / N );
		return out;
	}
	if ( N < 2 ) {
		out[ 0 ] = 0.0;
		out[ 1 ] = 0.0;
		return out;
	}
	out[ 0 ] = sqrt( M2x / (N-1) );
	out[ 1 ] = sqrt( M2y / (N-1) );
	return out;
}
Computes standard deviations.

@private
@param {Array} out - output array
@param {ArrayArray} arr - input array
@param {number} mx - `x` mean
@param {number} my - `y` mean
@param {boolean} bool - boolean indicating whether to compute a biased standard deviation
@returns {Array} output array