Added the entropy calculation

Author: mesonepigreco

Modules/Ensemble.py

@@ -1639,6 +1639,39 @@ def get_free_energy(self, return_error = False):
             return free_energy, error
         return free_energy
 
+    def get_entropy(self, return_error = False):
+        r"""
+        GET THE ENTROPY
+        ===============
+
+        Get the total anharmonic entropy.
+
+        The equation implemented is the analytical derivative of the free energy,
+        and assumes that the SSCHA free energy is minimized.
+
+        .. math::
+
+            S = - \frac{dF}{dT}
+
+            S = S_{harm} - \left<V - {\mathcal V}\right>\sum_\mu \frac{1}{1 + 2n_\mu} \frac{dn_\mu}{dT}
+
+
+        where :math:`S_{harm}` is the 'harmonic' entropy computed from the dynamucal matrix, 
+        plus a correction accounting for the ensemble anharmonicity.
+
+        
+        Parameters
+        ----------
+            return_error : bool
+                If true, returns also the error
+
+        Results
+        -------
+            entropy, error : float
+                Returns the entropy and [optionally] the stochastic error.
+        """
+        raise NotImplementedError("Error, to be implemented")
+
 
     def get_free_energy_interpolating(self, target_supercell, support_dyn_coarse = None, support_dyn_fine = None, error_on_imaginary_frequency = True, return_error = False):
         """