AP_BattMonitor: add static create method

libraries/AP_BattMonitor/AP_BattMonitor.h

@@ -37,9 +37,13 @@ class AP_BattMonitor
     friend class AP_BattMonitor_SMBus_Maxell;
 
 public:
+    static AP_BattMonitor create() { return AP_BattMonitor{}; }
 
-    /// Constructor
-    AP_BattMonitor();
+    constexpr AP_BattMonitor(AP_BattMonitor &&other) = default;
+
+    /* Do not allow copies */
+    AP_BattMonitor(const AP_BattMonitor &other) = delete;
+    AP_BattMonitor &operator=(const AP_BattMonitor&) = delete;
 
     // Battery monitor driver types
     enum BattMonitor_Type {
@@ -175,6 +179,8 @@ protected:
     AP_Int8     _low_voltage_source;                                /// voltage type used for detection of low voltage event
 
 private:
+    AP_BattMonitor();
+
     BattMonitor_State state[AP_BATT_MONITOR_MAX_INSTANCES];
     AP_BattMonitor_Backend *drivers[AP_BATT_MONITOR_MAX_INSTANCES];
     uint8_t     _num_instances;                                     /// number of monitors