Observation: introduce __hash__

More information here: https://docs.python.org/3/reference/datamodel.html#object.__hash__

resolve/data/observation.py

@@ -180,6 +180,10 @@ class BaseObservation:
             return False
         return compare_attributes(self, other, self._eq_attributes)
 
+    def __hash__(self):
+        # Since Observations are immutable objects this is okay
+        return id(self)
+
     @property
     def antenna_names(self):
         return list(self.auxiliary_table("ANTENNA")["NAME"])