Add support for NMEA 4.10 TAG Blocks in aisnmea convenience library

[simeonmiteff]

Hi Bertold!

We recently worked with the go-nmea maintainers to get an earlier PR merged to add NMEA 4.10 TAG Blocks support: adrianmo/go-nmea#78

This PR adds support for:

• Accessing the TAG Blocks info (which go-nmea now handles) via the aisnmea.VdmPacket struct (including some logic to merge tags from multi-sentence messages into one struct).
• Encoding the values from the same struct into NMEA sentences (but only if they are not blank values, so if you don't touch that struct, you'll get un-tagged NMEA sentences, hence no change for existing applications that use go-ais)

I have added unit tests for the TAG Block processing code and two tests for checking that end-to-end decoding still works. The existing unit tests still pass, so I don't think this PR breaks anything.

Thanks for go-ais, and please let me know if you have any questions, concerns or desire any changes to get this PR accepted.

Regards,
Simeon.