Unable to switch to guided mode

[EliotBR (Eliot)]

We’ll need some info from @williangalvani about the requirements here, because I haven’t looked into this part of the firmware before.

From some initial searching, it seems that

• AP_NavEKF3_VehicleStatus.cpp is where the GPS “goodness” checks are defined, where
  • frontend->_gpsCheckScaler comes from the EK3_CHECK_SCALE parameter
  • frontend->_gpsCheck comes from the EK3_GPS_CHECK parameter
• AHRS_GPS_MINSATS may also be relevant, although from the code it seems like ArduSub might currently just use (hardcoded) 6 (but maybe it’s used somewhere else in the EKF/AHRS code - I’m not sure)