AP_BattMonitor: unify singleton naming to _singleton and get_singleton()

libraries/AP_BattMonitor/AP_BattMonitor.cpp

@@ -238,7 +238,7 @@ AP_BattMonitor::read()
         }
     }
 
-    AP_Logger *df = AP_Logger::instance();
+    AP_Logger *df = AP_Logger::get_singleton();
     if (df->should_log(_log_battery_bit)) {
         df->Write_Current();
         df->Write_Power();
@@ -494,7 +494,7 @@ namespace AP {
 
 AP_BattMonitor &battery()
 {
-    return AP_BattMonitor::battery();
+    return *AP_BattMonitor::get_singleton();
 }
 
 };

libraries/AP_BattMonitor/AP_BattMonitor.h

@@ -54,8 +54,8 @@ public:
     AP_BattMonitor(const AP_BattMonitor &other) = delete;
     AP_BattMonitor &operator=(const AP_BattMonitor&) = delete;
 
-    static AP_BattMonitor &battery() {
-        return *_singleton;
+    static AP_BattMonitor *get_singleton() {
+        return _singleton;
     }
 
     struct cells {