handle empty data files when loading, closes #148 (#149)

Author: basnijholt

adaptive/learner/learner1D.py

@@ -654,7 +654,8 @@ def _get_data(self):
         return self.data
 
     def _set_data(self, data):
-        self.tell_many(*zip(*data.items()))
+        if data:
+            self.tell_many(*zip(*data.items()))
 
 
 def _fix_deepcopy(sorted_dict, x_scale):

adaptive/learner/learnerND.py

@@ -770,7 +770,8 @@ def _get_data(self):
         return self.data
 
     def _set_data(self, data):
-        self.tell_many(*zip(*data.items()))
+        if data:
+            self.tell_many(*zip(*data.items()))
 
     def _get_iso(self, level=0.0, which='surface'):
         if which == 'surface':