function meanpw( N, x, strideX, offsetX ) {
	if ( N <= 0 ) {
		return NaN;
	}
	return gsumpw( N, x, strideX, offsetX ) / N;
}
Computes the arithmetic mean of a strided array using pairwise summation.

@param {PositiveInteger} N - number of indexed elements
@param {NumericArray} x - input array
@param {integer} strideX - stride length
@param {NonNegativeInteger} offsetX - starting index
@returns {number} arithmetic mean

@example
var x = [ 2.0, 1.0, 2.0, -2.0, -2.0, 2.0, 3.0, 4.0 ];

var v = meanpw( 4, x, 2, 1 );
// returns 1.25