Missing support for DSC NMEA alerts and messages

[mgrouch]

Described here:

https://continuouswave.com/whaler/reference/DSC_Datagrams.html

More links:

See https://github.com/adrianmo/go-nmea/ (DSC, DSE)

https://www.cruisersforum.com/forums/f13/nmea-0183-sentences-dsc-dse-124887.html

https://continuouswave.com/whaler/reference/DSC_Datagrams.html

https://web.archive.org/web/20190303170916/http://continuouswave.com/whaler/reference/DSC_Datagrams.html

https://github.com/mariokonrad/marnav/blob/master/src/marnav/nmea/dsc.cpp

https://github.com/mariokonrad/marnav/blob/master/src/marnav/nmea/dse.cpp

Thanks