correlated_fields.py: Harden fluctuations property

Convert the return list of operators to a tuple as to disallow the
caller to amend or change it in other ways.

src/library/correlated_fields.py

@@ -756,7 +756,7 @@ class CorrelatedFieldMaker:
     @property
     def fluctuations(self):
         """Returns the added fluctuations operators used in the model"""
-        return self._a
+        return tuple(self._a)
 
     def get_normalized_amplitudes(self):
         """Returns the normalized amplitude operators used in the final model