fix: APB nextpoint paths, XTE in nautical miles

[sergei]

• Fixes APB sentence not emitted #61 by using navigation.courseGreatCircle.nextPoint.bearingTrue and navigation.courseGreatCircle.nextPoint.bearingMagnetic individually
• Change units of XTE to NM
• Make all degrees positive

[tkurki]

This is in line with.

• NMEA0183 to SK: https://github.com/SignalK/nmea0183-signalk/blob/6e2602c0570c7121f493a1c6ab9b29f8aa2b29c1/test/RMB.js#L42
• derived data: https://github.com/SignalK/signalk-derived-data/blob/9f1d9b87756fd809db19b08fc30ba7e8dda4bd64/calcs/courseData.js#L89
• N2K to SK: https://github.com/SignalK/n2k-signalk/blob/4b56bf44800df7e36b6b0a1bc3dea43b131caf0f/test/129284_navigation_data.js#L27

I would have preferred 2 commits, one for XTE and another for the APB fix, but what the heck, we've ignored this for way too long, apologies, so all I can say is thank you.