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.
API
new($num)
Creates a new XML::LibXML::Number object, with the value in $num. Does some
rudimentary numeric checking on $num to ensure it actually is a number.
value()
Also as overloaded stringification. Returns the numeric value held.
