AP_NavEKF2: make coreBetterScore const

libraries/AP_NavEKF2/AP_NavEKF2.cpp

@@ -758,7 +758,7 @@ bool NavEKF2::InitialiseFilter(void)
   return true if a new core index has a better score than the current
   core
  */
-bool NavEKF2::coreBetterScore(uint8_t new_core, uint8_t current_core)
+bool NavEKF2::coreBetterScore(uint8_t new_core, uint8_t current_core) const
 {
     const NavEKF2_core &oldCore = core[current_core];
     const NavEKF2_core &newCore = core[new_core];

libraries/AP_NavEKF2/AP_NavEKF2.h

@@ -555,7 +555,7 @@ private:
 
     // return true if a new core has a better score than an existing core, including
     // checks for alignment
-    bool coreBetterScore(uint8_t new_core, uint8_t current_core);
+    bool coreBetterScore(uint8_t new_core, uint8_t current_core) const;
     
     // logging functions shared by cores:
     void Log_Write_NKF1(uint8_t core, uint64_t time_us) const;