Fix meters to mm conversions

Add support for specifying fixed RTK base position

sequence numbers are only increased after a new RTCM message, while only
the fragment id is incremented within a fragmented message.

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

_finalize methods need information which is not available if all you
have is a mavlink_message_t

This avoids conflicts with other code