Fix oberservable names not being a list in Pyhton 3

jackknife.py

@@ -157,7 +157,7 @@ class Jackknife(object):
         print("-- Calculating transformed input mean ...")
         self._input_mean = sum(i for i in get_samples()) / n
         self._transformed_mean = f(self._input_mean)
-        self._observables_names = self._transformed_mean.keys()
+        self._observables_names = list(self._transformed_mean.keys())
         self._n_observables = len(self._observables_names)
         self._transformed_mean = list(self._transformed_mean.values())
         if isinstance(self.correlation_axis, int):