Extract NMEA messages from this package

[pjreed]

@kriskozak says:

While work is going on to add support for new messages, I would recommend looking into moving some or all of the messages out of this package. At the very least, the NMEA messages don't really belong here -- they have occasionally gotten used by other GPS drivers. Perhaps they should be added to https://github.com/swri-robotics/marti_messages

[evenator]

There's a pending PR to add the NMEA messages to nmea_msgs: ros-drivers/nmea_msgs#5

Once I accept that, we can remove those messages from this package.

[kriskozak]

Fyi @pjreed -- Looks like this is done.