bds b3i: Adding code to process BeiDou B3I signals

Adds code to perform acq and trk in BeiDou B3I signals. Stages of
telemetry decoding, observables computation and pvt use existing
code on the platform. Some further testing is required

src/core/receiver/gnss_flowgraph.h

@@ -185,6 +185,7 @@ private:
     std::list<Gnss_Signal> available_GLO_1G_signals_;
     std::list<Gnss_Signal> available_GLO_2G_signals_;
     std::list<Gnss_Signal> available_BDS_B1_signals_;
+    std::list<Gnss_Signal> available_BDS_B3_signals_;
     enum StringValue
     {
         evGPS_1C,
@@ -195,7 +196,8 @@ private:
         evGAL_5X,
         evGLO_1G,
         evGLO_2G,
-		evBDS_B1
+		evBDS_B1,
+		evBDS_B3
     };
     std::map<std::string, StringValue> mapStringValues_;