Improve `getGPSAntennaOffset()` in SurfaceOperationalStatusV2Msg

[fixje]

getGPSAntennaOffset() in SurfaceOperationalStatusV2Msg returns the encoded lateral and longitudinal offsets

1. Correct the Javadoc for this method
2. Add two convenience methods which return the offsets in meter according to the standard

[mrksngl]

I don't know what mood I was in here:

• V1 (as written in the first version of this message) has no such information
• the linked code was pointing to something completely different
• the javadoc is accurate enough, except that the tables of section 2 should be used over those in appendix A

So only the "add convenience methods" feature request remains.

[mrksngl]

Now I confused it completely: that issue was not by me but by @fixje .
But well...the corrects remain.

[mrksngl]

See also #52 which should be done first, making special cases more explicit