Support for generic meteorological bulletins

• Added new model classes for GenericMeteorologicalBulletin and GenericAviationWeatherMessage. These enable handling non-specific aviation messages and bulletins, with the raw message content (TAC or IWXXM) and some metadata, such as issue time, valid time and target aerodrome.
• TAFBulletinHeading and SIGMETBulletinHeading have been replaced with a generic BulletinHeading interface and the corresponding FreeBuilder implementation classes.
• fi.fmi.avi.model.GTSExchangeFileNameBuilder class is now renamed and refactored to fi.fmi.avi.util.GTSExchangeFileInfo.Builder. The new GTSExchangeFileInfo class now supports also parsing the BulletinHeading, timestamp etc. from the GTS exchange file name (or bulletinID).
• Issue time information moved from the MeteorologicalBulletin into BulletinHeading.
• It is now mandatory to give either a partial or complete time info for PartialOrCompleteTimeInstant instances.
• FreeBuilder version upgraded from 1.14.8 to 2.3.0