function mean( lambda ) {
	if ( isnan( lambda ) || lambda <= 0.0 ) {
		return NaN;
	}
	return 1.0 / expm1( lambda );
}
Returns the expected value of a Planck distribution.

@param {PositiveNumber} lambda - shape parameter
@returns {NonNegativeNumber} expected value

@example
var v = mean( 0.1 );
// returns ~9.5083

@example
var v = mean( 0.5 );
// returns ~1.5415

@example
var v = mean( 1.1 );
// returns ~0.49896

@example
var v = mean( -2.5 );
// returns NaN

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