add positional timestamp and geoidal separation

Relevant to issues #842 and #843 (timestamp) and historical issues #392 and #359 (timestamp, geoidal)

src/gps/GPS.h

@@ -45,6 +45,9 @@ class GPS : private concurrency::OSThread
                           // scaling before use)
     uint32_t heading = 0; // Heading of motion, in degrees * 10^-5
 
+    int32_t geoidal_height = 0;  // geoidal separation, in meters!
+    time_t pos_timestamp = 0;  // positional timestamp from GPS solution
+
     GPS() : concurrency::OSThread("GPS") {}
 
     virtual ~GPS();