Error on missing middle reflectance sensor

Add a runtime check in Reflectance.get_middle that raises RuntimeError when the middle reflectance pin is not supported on the board. Update the docstring to document the raised error and clarify the return value (reflectance 0..1). This prevents calling _get_value with a None sensor and gives a clearer failure mode for unsupported hardware configurations.

Author: SaintSampo

XRPLib/reflectance.py

@@ -52,10 +52,14 @@ def get_left(self) -> float:
 
     def get_middle(self) -> float:
         """
-        Gets the the reflectance of the left reflectance sensor
-        : return: The reflectance ranging from 0 (white) to 1 (black)
+        Gets the reflectance of the middle sensor if available.
+        :raises RuntimeError: If the middle pin is not supported on this board.
+        :return: Reflectance from 0 (white) to 1 (black)
         : rtype: float
         """
+        if self._middleReflectance is None:
+            raise RuntimeError(f"Middle reflectance sensor is not available on this board configuration.")
+        
         return self._get_value(self._middleReflectance)
 
     def get_right(self) -> float: