Added two human readable examples

Author: mesonepigreco

Modules/Relax.py

@@ -277,8 +277,7 @@ def relax(self, restart_from_ens = False, get_stress = False,
                 with some ase calculator)
             ensemble_loc : string
                 Where the ensemble of each population is saved on the disk. If none, it will
-                use the content of self.data_dir. If also self.data_dir is None, 
-                the ensemble will not not be saved (useful to avoid disk I/O for force fields)
+                use the content of self.data_dir. It is just a way to override the variable self.data_dir
             start_pop : int, optional
                 The starting index for the population, used only for saving the ensemble and the dynamical 
                 matrix. If None, the content of self.start_pop will be used.
@@ -292,6 +291,27 @@ def relax(self, restart_from_ens = False, get_stress = False,
 
         if ensemble_loc is None:
             ensemble_loc = self.data_dir
+
+        if (not ensemble_loc) and self.save_ensemble:
+            ERR_MSG = """
+Error, you must specify where to save the ensembles.
+       this can be done either passing ensemble_loc = "path/to/dir" 
+       for the ensemble, or by setting the data_dir attribute of this object.
+"""
+            raise IOError(ERR_MSG)
+        
+        if self.save_ensemble:
+            if not os.path.exists(ensemble_loc):
+                os.makedirs(ensemble_loc)
+            else:
+                if not os.isdir(ensemble_loc):
+                    ERR_MSG = """
+Error, the specified location to save the ensemble:
+       '{}' 
+       already exists and it is not a directory.
+""".format(ensemble_loc)
+                    raise IOError(ERR_MSG)
+
 
         if start_pop is None:
             start_pop = self.start_pop