function ratioArray( X ) {
	var R;
	var i;

	R = [];
	for ( i = 0; i < X.length-1; i++ ) {
		R.push( X[ i+1 ] / X[ i ] );
	}
	return R;
}
Returns an array containing the ratio of each pair of consecutive elements in order: `X[ i+1 ] / X[ i ]`.

@private
@param {NumberArray} X - input array
@returns {NumberArray} ratio array

@example
var R = ratioArray( [ 1.0, 2.0, 5.0 ] );
// returns [ 2.0, 2.5 ]