Bulletin serializer refactoring, GenericMeteorologicalBulletin parsing

• Added support for parsing WMO Collect / IWXXM bulletins into GenericMeteorologicalBulletin POJOs. Any IWXXM 2.1 contained messages are allowed, but the message metadata (issue & valid time, aerodrome) is only parsed from TAFs and SIGMETs for now.
• TimeInstant valued phenomenon times are now correctly handled for METAR/SPECI TRENDs, (issue #19)
• Moved all bulletin parsing and serialization classes to package fi.fmi.avi.converter.iwxxm.bulletin
• Various property container classes used for passing the parsed object values from the scanners to parsers no longer have a mandatory reference to the JAXB parent instance. Empty constructors must be used instead.
• Fixed the invalid bulletin identifier format (missing issue time)