SIGMETBulleting support, pass-through TAC SIGMET

• Added a temporary SIGMET message type capable in including only the TAC string SIGMET content. This was required for producing text-based SIGMET bulletins. The SIGMET class will probably be replaced with the actual SIGMET message type class in the next release.
• Added the SIGMETBulletin container message type.
• Added JSON <-> POJO conversion support for TAFBulletin, SIGMETBulletin and SIGMET
• Code style and bug fixes