implement reset_tuning on Metropolis

closes pymc-devs#3733

pymc3/step_methods/metropolis.py

@@ -146,12 +146,25 @@ def __init__(self, vars=None, S=None, proposal_dist=None, scaling=1.,
         self.any_discrete = self.discrete.any()
         self.all_discrete = self.discrete.all()
 
+        # remember initial settings before tuning so they can be reset
+        self._untuned_settings = dict(
+            scaling=self.scaling,
+            steps_until_tune=tune_interval,
+            accepted=self.accepted
+        )
+
         self.mode = mode
 
         shared = pm.make_shared_replacements(vars, model)
         self.delta_logp = delta_logp(model.logpt, vars, shared)
         super().__init__(vars, shared)
 
+    def reset_tuning(self):
+        """Resets the tuned sampler parameters to their initial values."""
+        for attr, initial_value in self._untuned_settings.items():
+            setattr(self, attr, initial_value)
+        return
+
     def astep(self, q0):
         if not self.steps_until_tune and self.tune:
             # Tune scaling parameter