Checksum handling #124
Description
The https://github.com/ais-dotnet/Ais.Net.Receiver project has ended up implementing its own checksum generation. It uses this when appending timestamp headers on messages that don't come with a timestamp. (Some AIS stations don't seem to include an NMEA header.) This seems like a capability that should really be built into this library.
Also, review how much checksum verification we actually do in this library.