Generic class XMLDateTime with optional components

[MenoData]

This would roughly model what XMLGregorianCalendar also does but with a nicer API and higher time resolution (nanosecs versus millisecs).

It can contain methods like hasDate(), getDate(), getOffset() etc. to query optional components. There should also be parsing and formatting methods for all standard XML-Schema types like xsd:dateTime, xsd:gMonthDay etc.

This enhancement concerns the misc-module (net.time4j.xml-package).