RINEX printer now takes objects by reference instead of by value. Adding

some const.

src/core/system_parameters/gps_navigation_message.h

@@ -15,7 +15,7 @@
  * GNSS-SDR is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
  * the Free Software Foundation, either version 3 of the License, or
- * at your option) any later version.
+ * (at your option) any later version.
  *
  * GNSS-SDR is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ -231,7 +231,7 @@ public:
      * \brief Computes the Coordinated Universal Time (UTC) and
      * returns it in [s] (IS-GPS-200E, 20.3.3.5.2.4)
      */
-    double utc_time(double gpstime_corrected);
+    double utc_time(const double gpstime_corrected) const;
 
     bool satellite_validation();