SGMetar: avoid raw char pointers in the public API

Ideally would refactor to remove the raw pointers internally, but this
is a good first step.

simgear/environment/metar.cxx

@@ -491,9 +491,17 @@ void SGMetar::useCurrentDate()
 	_month = now.tm_mon + 1;
 }
 
-const char* SGMetar::getRawDataPtr()
+std::string SGMetar::getDataString() const
 {
-    return _data.data();
+    return std::string{_data.data()};
+}
+
+std::string SGMetar::getUnparsedData() const
+{
+    if (!_m || (strlen(_m) < 1))
+        return {};
+
+    return std::string{_m};
 }
 /**
   * Replace any number of subsequent spaces by just one space, and add

simgear/environment/metar.hxx

@@ -186,7 +186,8 @@ public:
 		std::vector<std::string> phenomena;
 	};
 
-    const char* getRawDataPtr();
+    std::string getDataString() const;
+    std::string getUnparsedData() const;
 
     inline       bool getProxy()		const { return _x_proxy; }
 	inline const char *getId()		const { return _icao; }