SFY variant with external UBLOX GPS

[gauteh]

Implements #170 .

[gauteh]

sparkfun/SparkFun_u-blox_GNSS_v3#67

[gauteh]

@edwinrainville (and also @jerabaul29 if you are interested? :)), I'm trying to think of a sub-millisecond sync algorithm:

• Ublox PVT updated ( 20 Hz)
• Ublox PPS -> ext-gps-mod
• pps_time = ext-gps-mod.micros()
• ext-gps-mod PPS -> SFY
• SFY: stores sfy_pps_time: for incoming PVT telegram
• read_time = ext-gps-mod.micros()
• ext-gps-mod reads PVT related to PPS. Is the T updated or is it fixed to the
  PPS? maybe all of this is fast enough to ignore.
• ext-gps-mod push PVT to SFY + (read_time - pps_time)
• sfy_read_time = sfy.micros()/RTC
• SFY reads PVT related to PPS
• SFY sets local time to T + (sfy_read_time - sfy_pps_time)

Does that make sense? I have PPS working on the arduino-code, not yet on SFY. We need the synchronization to match IMU and GPS timeseries.

https://github.com/gauteh/sfy/tree/sfy-rtk/sfy-buoy/sfy-ext-gps#sfy-with-rtk-relay

[gauteh]

According to 12.1.8 the maximum jitter of the RTC is 1 ms for XR crystal, or 8 ms for LFRC, which is good enough.