fix Pickleability

Author: jpivarski

histogrammar/defs.py

@@ -151,6 +151,11 @@ def fill(self, datum, weight=1.0):
         """
         raise NotImplementedError
 
+    def __getstate__(self):
+        state = dict(self.__dict__)
+        del state["fill"]
+        return state
+
     def copy(self):
         """Copy this container, making a clone with no reference to the original. """
         return self + self.zero()