Remove check for nav_sat_stat and nav_sat_cov from nav_sat_fix node

[gsokoll]

Proposed change to address #13

Existing code for the nav_sat_fix node checked whether updated values for both nav_sat status and covariance data had been received when processing incoming ubx_nav_hpposllh messages. The ublox device should normally publish the status and covariance data at the same rate as the hpposllh data, however when the io systm was becoming overloaded these messages appear to be dropped. This resulted in the nav_sat_fix messages being published very slowly.

This change removes the check for updated status and covariance data. Instead it uses the last known values for them when preparing the nav_sat_fix message.