Add fn:current-date-time(), fn:current-date() and fn:current-time()

[mbeckerle]

A requirement has been received for the DFDL serializer can insert the current date/time into an output field. This would require the addition of the XPath functions fn:current-date-time(), fn:current-date() and fn:current-time() to the DFDL XPath 2.0 subset, which could then be used in a dfdl:outputValueCalc expression.