function mean( out, arr ) {
	var delta;
	var mx;
	var my;
	var N;
	var i;

	mx = 0.0;
	my = 0.0;

	N = 0;
	for ( i = 0; i < arr.length; i++ ) {
		N += 1;
		delta = arr[i][0] - mx;
		mx += delta / N;
		delta = arr[i][1] - my;
		my += delta / N;
	}
	out[ 0 ] = mx;
	out[ 1 ] = my;
	return out;
}
Computes sample means using Welford's algorithm.

@private
@param {Array} out - output array
@param {ArrayArray} arr - input array
@returns {Array} output array