added Combinations enum to gnss_utils.h

include/gnss_utils/gnss_utils.h

@@ -36,6 +36,11 @@ typedef std::shared_ptr<const Navigation>   NavigationConstPtr;
 typedef std::shared_ptr<Snapshot>           SnapshotPtr;
 typedef std::shared_ptr<const Snapshot>     SnapshotConstPtr;
 
+enum Combination
+{
+    CODE_L1,    // only L1 code
+    CARRIER_L1  // only L1 carrier phase
+};
 
 }