Add template casting to readValue and advertisement data (#52)

The value returned by reading a remote characteristic or by getting a notification for it is kept in the class instance of the NimBLERemoteCharacteristic. This value can be accessed as a std::string type using the getValue() function.

This adds templates to read the value in the type used by the peripheral. The same functionality is implemented for getting the manufacturer data or the service data of an advertised device.

src/NimBLEAdvertisedDevice.h

@@ -45,10 +45,28 @@ public:
     uint8_t         getAdvType();
     uint16_t        getAppearance();
     std::string     getManufacturerData();
+
+    template<typename T>
+    T               getManufacturerData(bool skipSizeCheck = false) {
+        std::string data = getManufacturerData();
+        if(!skipSizeCheck && data.size() < sizeof(T)) return T();
+        const char *pData = data.data();
+        return *((T *)pData);
+    }
+
     std::string     getName();
     int             getRSSI();
     NimBLEScan*     getScan();
     std::string     getServiceData();
+
+    template<typename T>
+    T               getServiceData(bool skipSizeCheck = false) {
+        std::string data = getServiceData();
+        if(!skipSizeCheck && data.size() < sizeof(T)) return T();
+        const char *pData = data.data();
+        return *((T *)pData);
+    }
+
     NimBLEUUID      getServiceDataUUID();
     NimBLEUUID      getServiceUUID();
     int8_t          getTXPower();