WString: add `toDouble`

`toFloat` internally converts into double and then truncates into a
float, so why not add a method to return the double?

cores/arduino/WString.cpp

@@ -740,6 +740,11 @@ long String::toInt(void) const
 
 float String::toFloat(void) const
 {
-	if (buffer) return float(atof(buffer));
+	return float(toDouble());
+}
+
+double String::toDouble(void) const
+{
+    if (buffer) return atof(buffer);
     return 0;
 }

cores/arduino/WString.h

@@ -190,6 +190,7 @@ public:
 	// parsing/conversion
 	long toInt(void) const;
 	float toFloat(void) const;
+    double toDouble(void) const;
 
 protected:
 	char *buffer;	        // the actual char array