this changes yaw handling in a few ways:
- GPS yaw now has a timestamp associated with the yaw separate from
the timestamp associated with the GPS fix
- we no longer force the primary to change to the UBLOX MB rover when
it has a GPS yaw. This means we don't change GPS primary due to GPS
loss, which keeps the GPS more stable. It also increases accuracy
as the rover is always less accurate in position and velocity than
the base
- now we force the primary to be the MB base if the other GPS is a
rover and the base has GPS lock
