function mean( v ) {
	return addon( v );
}
Evaluates the mean for a t-distribution with degrees of freedom `v`.

@private
@param {number} v - degrees of freedom
@returns {number} mean

@example
var y = mean( 10.0 );
// returns 0.0

@example
var y = mean( 2.0 );
// returns 0.0

@example
var y = mean( NaN );
// returns NaN