Simple numeric values.
This class holds simple numeric values. It doesn't support -0, +/- Infinity,
or NaN, as the XPath spec says it should, but I'm not hurting anyone I don't think.
Creates a new Tree::XPathEngine::Number object, with the value in $num. Does some
rudimentary numeric checking on $num to ensure it actually is a number.
Also as overloaded stringification. Returns the numeric value held.
Those are needed so the objects can be properly processed in various contexts