function mean( N, x, strideX ) {
	return meanpn( N, x, strideX );
}
Computes the arithmetic mean of a strided array.

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

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

var v = mean( x.length, x, 1 );
// returns ~0.3333