Enhancement: Multiply unit by numeric factor

[neilcrookes]

E.g.

/* You can multiply a unit by calling `multiply` method with a float as an argument
 * add always returns a new instance
 */
$quantity = 7;
$unitWeight = new Mass\Gram(500);
$totalWeight = $unitWeight->multiply($quantity);
echo $totalWeight->format(0);
// 3500 g