function epanechnikov( u ) {
	if ( isnan( u ) ) {
		return NaN;
	}
	if ( abs( u ) > 1.0 ) {
		return 0.0;
	}
	return 0.75 * ( 1.0 - pow( u, 2.0 ) );
}
Computes the Epanechnikov kernel for a value.

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

@example
var u = 5.0;
var out = epanechnikov( u );
// returns 0.0