feat(AveragePosition/GetNTRIPMountPoint): change return type, update cmd average_position

reach/client/protocol/average_postion.go

@@ -0,0 +1,12 @@
+package protocol
+
+type Coordinates struct {
+	Latitude  float64 `json:"latitude"`
+	Longitude float64 `json:"longitude"`
+	Height    float64 `json:"height"`
+}
+
+type AveragePositionPayload struct {
+	Coordinates   Coordinates `json:"coordinates"`
+	AntennaOffset float64     `json:"antenna_offset"`
+}