DigitalOutputProtocol: add method to invert the value of a digital output

The "invert" method should invert the value of a digital output.

Signed-off-by: Perry Melange <[email protected]>

Author: pmelange

labgrid/protocol/digitaloutputprotocol.py

@@ -13,3 +13,7 @@ def get(self):
     def set(self, status):
         """Implementations should set the status of the digital output"""
         raise NotImplementedError
+
+    @abc.abstractmethod
+    def invert(self):
+        """Implementations should invert the the status of the digital output"""