function cosine( u ) {
	if ( isnan( u ) ) {
		return NaN;
	}
	if ( abs( u ) > 1.0 ) {
		return 0.0;
	}
	return FOURTH_PI * cos( HALF_PI * u );
}
Computes the cosine kernel for a value.

@private
@param {number} u - value for which we wish to calculate the cosine kernel
@returns {number} the value for the kernel at u

@example
var u = 5;
var out = cosine( u );
// returns 0.0