Provide vessel speed over ground when available

[gavinmacaulay]

I miss the speed_over_ground variable when reading in Simrad EK data files - is it somewhere? A search of the echopype code didn't show up any code dealing with extracting speed from NMEA data, so perhaps not?

The sonar-netcf4 convention has 'speed_over_ground' as a variable under Platform (v1) or under Platform/NMEA (v2).